We relied on them being valid longer to keep track of used GPUs. If we
don't have the CRTC (or output) we don't have a way to fetch the pointer
to the MetaGpu that drives the associated monitor.
This avoids a crash when trying to fetch said pointer from what would be
the NULL MetaCrtc pointer.
Fixes: 08593ea872 ("onscreen/native: Hold ref to the output and CRTC until detached")
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2667
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2887>
