wayland: Fix subsurface place_above/below type cast error

A MetaWaylandSurface was casted into a ClutterActor, but it should have
been the MetaSurfaceActor.

Move out parent_actor and surface_actor out of the loop while at it
since they won't change when iterating.

https://bugzilla.gnome.org/show_bug.cgi?id=745655