drm/amd/display: fix NULL checks for adev->dm.dc in amdgpu_dm_fini()

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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"
                    ],
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                            "version": "6.9",
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                }