backends: Add flags to MetaMonitorMode

And export them in the DBus API since they're useful for
gnome-control-center.

https://bugzilla.gnome.org/show_bug.cgi?id=763832

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

@@ -632,6 +632,7 @@ meta_monitor_manager_kms_read_current (MetaMonitorManager *manager)
       meta_mode->name = g_strndup (mode->name, DRM_DISPLAY_MODE_LEN);
       meta_mode->width = mode->hdisplay;
       meta_mode->height = mode->vdisplay;
+      meta_mode->flags = mode->flags;
 
       /* Calculate refresh rate in milliHz first for extra precision. */
       meta_mode->refresh_rate = (mode->clock * 1000000LL) / mode->htotal;