display: only add user authorization if connected to display
    
    If we aren't connected to the display then we can't give
    the user access to it,(and we don't need to anyway)
    
    This commit adds a new is-connected property to GdmDisplay and
    changes the code to never give a user authorization if we
    aren't connected.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=744764