[media] v4l: vsp1: Add support for the RPF alpha multiplier on Gen3

The Gen3 RPF includes an alpha multiplier that can both multiply the
alpha channel by a fixed global alpha value, and multiply the pixel
components to convert the input to premultiplied alpha.

As alpha premultiplication is available in the BRU for both Gen2 and
Gen3 we handle it there and use the Gen3 alpha multiplier for global
alpha multiplication only. This prevents conversion to premultiplied
alpha if no BRU is present in the pipeline, that use case will be
implemented later if needed.

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