[media] vpif_display: use a v4l2_subdev pointer to call a subdev

This makes it easier to have outputs without subdevs.
This needs more work. The way the outputs are configured should be identical
to how inputs are configured.

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>