drm/amd/display: Fix potential null dereference

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix potential null dereference The adev->dm.dc pointer can be NULL and dereferenced in amdgpu_dm_fini() without checking. Add a NULL pointer check before calling dc_dmub_srv_destroy(). Found by Linux Verification Center (linuxtesting.org) with SVACE.