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

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.

Cc: <stable@vger.kernel.org> # 6.18.x
Fixes: 73cfd166e045 ("drm/gud: Replace simple display pipe with DRM atomic helpers")
Signed-off-by: Shenghao Yang <me@shenghaoyang.info>
Reviewed-by: Ruben Wauters <rubenru09@aol.com>
Signed-off-by: Ruben Wauters <rubenru09@aol.com>
Link: https://patch.msgid.link/20251231055039.44266-1-me@shenghaoyang.info