drm/amd/display: Check null pointers before used

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/gpu/drm/amd/display/dc/clk_mgr/dce110/dce110_clk_mgr.c",
                        "drivers/gpu/drm/amd/display/dc/hubp/dcn10/dcn10_hubp.c",
                        "drivers/gpu/drm/amd/display/dc/hubp/dcn20/dcn20_hubp.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dce110/dce110_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn31/dcn31_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c"
                    ],
                    "versions": [
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "348f8668dfa77294a277946cb0231c29b5474e1d",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "692e9a20018da7aa225d8dced44803f9852aca81",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "5b35bf1a82eb29841b67ff5643ba83762250fc24",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "be1fb44389ca3038ad2430dac4234669bc177ee3",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/gpu/drm/amd/display/dc/clk_mgr/dce110/dce110_clk_mgr.c",
                        "drivers/gpu/drm/amd/display/dc/hubp/dcn10/dcn10_hubp.c",
                        "drivers/gpu/drm/amd/display/dc/hubp/dcn20/dcn20_hubp.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dce110/dce110_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn31/dcn31_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c"
                    ],
                    "versions": [
                        {
                            "version": "4.15",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "4.15",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.191",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.160",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.11.3",
                            "lessThanOrEqual": "6.11.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.12",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }