monitor-config-manager: Mark the mirroring logical monitor as primary

The resulting logical monitor was eventually marked as primary anyway,
but without the config being marked as such, various primary properties
was not set e.g. the one on the MetaOutput. Also, tests would fail.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2479>

src/backends/meta-monitor-config-manager.c

@@ -1275,7 +1275,8 @@ create_for_switch_config_all_mirror (MetaMonitorConfigManager *config_manager)
       .height = height
     },
     .scale = best_scale,
-    .monitor_configs = monitor_configs
+    .monitor_configs = monitor_configs,
+    .is_primary = TRUE,
   };
 
   logical_monitor_configs = g_list_append (NULL, logical_monitor_config);