[media] rcar-vin: rework how subdevice is found and bound

The original drivers code to find a subdevice by looking in the DT grpah
and how the callbacks to the v4l2 async bind framework where poorly
written. The most obvious example of badness was the duplication of data
in the struct rvin_graph_entity.

This patch removes the data duplication, simplifies the parsing of the
DT graph and add checks to the v4l2 callbacks.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>