[media] v4l: vsp1: Add Z-order support for DRM pipeline

Make the Z-order of planes configurable by assigning RPFs to BRU inputs
dynamically based on the Z-order position.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>