[media] omap3isp: Move media_entity_cleanup() from unregister() to cleanup()

The media_entity_cleanup() function belong to the module cleanup
handlers, not the entity registration handlers. Move it there.

Create a omap3isp_video_cleanup() function to cleanup the video node
entity, and call it from the module cleanup handlers.

Rename omap3isp_stat_free() to omap3isp_stat_cleanup().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>