[media] omap3isp: Find source pad from external entity

No longer assume pad number 0 is the source pad of the external entity. Find
the source pad from the external entity and use it instead.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>