[media] media: omap3isp: fix a potential NULL deref

Fix a potential NULL pointer dereference by skipping registration of
external entities in case none are provided.

This is useful at least when testing mere memory-to-memory scenarios.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>