[media] v4l: vsp1: Simplify alpha propagation

We don't need to walk the pipeline when propagating the alpha value as
all the information needed for propagation is already available from the
pipeline structure.

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