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

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: fix NULL checks for adev->dm.dc in amdgpu_dm_fini() Since 'adev->dm.dc' in amdgpu_dm_fini() might turn out to be NULL before the call to dc_enable_dmub_notifications(), check beforehand to ensure there will not be a possible NULL-ptr-deref there. Also, since commit 1e88eb1b2c25 ("drm/amd/display: Drop CONFIG_DRM_AMD_DC_HDCP") there are two separate checks for NULL in 'adev->dm.dc' before dc_deinit_callbacks() and dc_dmub_srv_destroy(). Clean up by combining them all under one 'if'. Found by Linux Verification Center (linuxtesting.org) with static analysis tool SVACE.

References

• https://git.kernel.org/stable/c/e040f1fbe9abae91b12b074cfc3bbb5367b79811
• https://git.kernel.org/stable/c/ca2eb375db76fd50f31afdd67d6ca4f833254957
• https://git.kernel.org/stable/c/1c62697e4086de988b31124fb8c79c244ea05f2b
• https://git.kernel.org/stable/c/2a3cfb9a24a28da9cc13d2c525a76548865e182c