drm/amd/display: Check null pointers before using them

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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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                        "drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c"
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{
                    "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/amdgpu_dm/amdgpu_dm.c"
                    ],
                    "versions": [
                        {
                            "version": "4.15",
                            "status": "affected"
                        },
                        {
                            "version": "0",
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                            "version": "6.11.3",
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                        {
                            "version": "6.12",
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                            "versionType": "original_commit_for_fix"
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                    ]
                }