simple-egl: use roundtrip after get_registry request

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>