[media] staging: media: omap4iss: Cleanup media entities after unregistration

The ipipeif, ipipe and resizer media entities are cleaned up before
unregistering the media device, creating a race condition. Fix it by
cleaning them up at cleanup time.

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