[media] omap3isp: Refactor collecting information on entities in pipeline

Collect information on entities in pipeline in isp_video_far_end(), outside
pipeline validation. As this causes the function to have side effects,
rename the function accordingly.

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