[media] v4l: vsp1: Add global alpha support for DRM pipeline

Make the global alpha multiplier of DRM planes configurable. All the
necessary infrastructure is there, we just need to store the alpha value
passed through the DRM API.

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