CVE-2023-53498 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 amdgpudmfini without checking. Add a NULL pointer check before calling dcdmubsrvdestroy. Found by Linux Verification Center...