crtc: Move logical monitor pointer to MetaMonitor

Since the last code fetching the logical monitor state directly from the
CRTC has been removed, we can move the logical monitor pointer to a more
natural place.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1042

src/backends/native/meta-monitor-manager-kms.c

@@ -221,8 +221,6 @@ apply_crtc_assignments (MetaMonitorManager *manager,
         {
           MetaCrtc *crtc = k->data;
 
-          crtc->logical_monitor = NULL;
-
           if (crtc->is_dirty)
             {
               crtc->is_dirty = FALSE;