x11: displays: Delay RRScreenChangeNotify handling

This is yet another in a series of commits concerning the handling of the RRScreenChangeNotify event (the latest of which was eb3ca062), which causes so many problems with configuration resets and/or screens that refuse to turn off (that come out of sleep as soon as they're put to sleep). I'm now confronted with this latter problem (perhaps as a result of a kernel or driver change), and the only way I can see to get around it without completely disabling any action on connection or disconnection of a new screen, is to defer processing of the RRScreenChangeNotify event, so that xrandr's internal machinations on screen shutdown are transparent to us.