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