drm/gud: fix NULL fb and crtc dereferences on USB disconnect

In the Linux kernel, the following vulnerability has been resolved: drm/gud: fix NULL fb and crtc dereferences on USB disconnect On disconnect drm_atomic_helper_disable_all() is called which sets both the fb and crtc for a plane to NULL before invoking a commit. This causes a kernel oops on every display disconnect. Add guards for those dereferences.