[media] media: Entity graph traversal

Add media entity graph traversal. The traversal follows enabled links by
depth first. Traversing graph backwards is prevented by comparing the next
possible entity in the graph with the previous one. Multiply connected
graphs are thus not supported.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vimarsh Zutshi <vimarsh.zutshi@gmail.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>