OMAPFB: fix releasing overlays

omapfb disables all the overlays when freeing resources, but it should
also remove those overlays from overlay managers.

Not doing so causes a crash if omapfb is unbound and bound, or omapfb
module is removed and loaded, while keeping omapdss around.

Fix this by calling unset_manager() for all overlays.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>