legacy-monitor-config: Init is_underscanning config field from output

Don't loose the is_underscanning field when creating a new configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-config.c

@@ -1119,6 +1119,7 @@ init_config_from_preferred_mode (MetaOutputConfig *config,
   config->transform = META_MONITOR_TRANSFORM_NORMAL;
   config->is_primary = FALSE;
   config->is_presentation = FALSE;
+  config->is_underscanning = output->is_underscanning;
 }
 
 /* This function handles configuring the outputs when the driver provides a