backend: Get 'is-stage-views-scaled' from backend

It did, but used the old backend singleton.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>

src/backends/meta-settings.c

@@ -98,7 +98,7 @@ update_ui_scaling_factor (MetaSettings *settings)
 {
   int ui_scaling_factor;
 
-  if (meta_is_stage_views_scaled ())
+  if (meta_backend_is_stage_views_scaled (settings->backend))
     ui_scaling_factor = 1;
   else
     ui_scaling_factor = calculate_ui_scaling_factor (settings);