[media] vpif_display: move output_id to channel_obj

The output index does not belong to video_obj, it belongs to
channel_obj. Also rename to output_idx to be consistent with
the input_idx name used in vpif_capture.

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>