display: ignore display that do not have toplevel window
    
    virt_viewer_display_get_preferred_monitor_geometry() may be called
    during application initialization (when the VTE console is being
    shown, virt_viewer_session_update_displays_geometry() is called when
    the visibility menu item is toggled). But the other displays may not
    yet be associated with a window, ignore them.
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>