simple-egl: use roundtrip after get_registry request

Sometimes weston-simple-egl aborts in create_surface
under some conditions. It is because wl_display_dispatch()
may not be enough to make sure we have all requried objects.
Can be modeled by wldbg:

  $ wldbg -i weston-simple-egl
  (wldbg) b re get_registry
  (wldbg) c
  (wldbg) c

After these steps the weston-simple-egl aborts, because
it has not got shell neither ivi-shell objects

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/simple-egl.c

@@ -861,7 +861,7 @@ main(int argc, char **argv)
 	wl_registry_add_listener(display.registry,
 				 &registry_listener, &display);
 
-	wl_display_dispatch(display.display);
+	wl_display_roundtrip(display.display);
 
 	init_egl(&display, &window);
 	create_surface(&window);