kms: Notify about privacy screen changes via predictions

When we change the privacy screen, we added a result listener to the KMS
update object to notify the upper layer about the privacy screen state
change. This was slightly awkward as one might have changed the state
multiple times for a single update, thus it was necessary to remove any
old result listeners to an update before adding a new one.

Doing this will not be possible when updates are fully async and managed
by the KMS impl device.

To handle this, instead make the post-commit prediction notify about
changes that happens in response to a successfully committed update. We
already predicted the new privacy screen state, so the necessary change
was to plumb the actual change into a callback which emits the signal if
there actually was a privacy screen change.

This will then be communicated via the same signal listener that already
listens to the 'resources-changed' signal.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2340>

src/backends/meta-monitor-manager-private.h

@@ -246,9 +246,6 @@ struct _MetaMonitorManagerClass
                            unsigned short     *green,
                            unsigned short     *blue);
 
-  gboolean (* set_privacy_screen_enabled) (MetaMonitorManager *manager,
-                                           gboolean            enabled);
-
   void (* tiled_monitor_added) (MetaMonitorManager *manager,
                                 MetaMonitor        *monitor);