On_layer of ivi_layout_surface should be NULL in the layer_destroy.
Previous code might access to removed ivilayer and cause SEGV.

Signed-off-by: Wataru Natsume <wataru_natsume@xddp.denso.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>