platformScreenForWindow() is currently implemented as:
	return window->screen()->handle();

Use window.screen() directly and avoid the QHighDpi
conversion call.

Change-Id: I0f6487efc4d1494813179665b21150f3ca7a8666
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>