Set enabled status of all displays when we get a monitor update
    
    Previously, when we received a new monitors update from the server, we
    only called virt_viewer_display_set_enabled() for the displays that were
    enabled. We simply assumed that those that were not enabled were already
    set to disabled. This assumption is currently valid, but I have some
    changes in the pipeline where this is not true. This change ensures that
    we update the enabled state of all monitors when we get an updated
    monitors conifguration.