[media] vpif_capture: separate subdev from input

vpif_capture relied on a 1-1 mapping of input and subdev. This is not
necessarily the case. Separate the two. So there is a list of subdevs
and a list of inputs. Each input refers to a subdev and has routing
information. An input does not have to have a subdev.
The initial input for each channel is set to the fist input.
Currently missing is support for associating multiple subdevs with
an input.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Tested-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>