monitor-manager: Get monitor info scale from main output tile

We didn't ever set the monitor info scale for tiled outputs; lets do
that by taking it from the main output.

https://bugzilla.gnome.org/show_bug.cgi?id=777470

src/backends/meta-monitor-manager.c

@@ -135,6 +135,7 @@ construct_tile_monitor (MetaMonitorManager *manager,
           info.width_mm = output->width_mm;
           info.height_mm = output->height_mm;
           info.winsys_id = output->winsys_id;
+          info.scale = output->scale;
         }
 
       /* hack */