[media] media: entity: Add debug information to graph walk

Use dev_dbg() to tell about the progress of the graph traversal algorithm.
This is intended to make debugging of the algorithm easier.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>