[media] rcar-vin: move subdev source and sink pad index to rvin_graph_entity

It makes more sense to store the sink and source pads in struct
rvin_graph_entity since that contains other subdevice related
information.

The data type to store pad information in is unsigned int and not int,
change this. While we are at it drop the _idx suffix from the names,
this never made sense.

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