drm/amd/display: Check null pointers before multiple uses

{
                    "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/core/dc_hw_sequencer.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/link/accessories/link_dp_cts.c",
                        "drivers/gpu/drm/amd/display/dc/link/hwss/link_hwss_dio.c",
                        "drivers/gpu/drm/amd/display/dc/resource/dce112/dce112_resource.c",
                        "drivers/gpu/drm/amd/display/dc/resource/dcn32/dcn32_resource.c",
                        "drivers/gpu/drm/amd/display/dc/resource/dcn32/dcn32_resource_helpers.c"
                    ],
                    "versions": [
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "7dc67992b35bc11895149b4261c4ad745312a18d",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "4f51f6d23c5856f105110531f81d84e78e462005",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "26787fb6c2b2ee0d1a7e1574b36f4711ae40fe27",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "fdd5ecbbff751c3b9061d8ebb08e5c96119915b4",
                            "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/core/dc_hw_sequencer.c",
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c",
                        "drivers/gpu/drm/amd/display/dc/link/accessories/link_dp_cts.c",
                        "drivers/gpu/drm/amd/display/dc/link/hwss/link_hwss_dio.c",
                        "drivers/gpu/drm/amd/display/dc/resource/dce112/dce112_resource.c",
                        "drivers/gpu/drm/amd/display/dc/resource/dcn32/dcn32_resource.c",
                        "drivers/gpu/drm/amd/display/dc/resource/dcn32/dcn32_resource_helpers.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"
                        }
                    ]
                }