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

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.