media: vsp1: Add premultiplied alpha support

To support DRM blend mode in R-Car DU driver, we must be able to pass
a plane with the premultiplied alpha. Adding a new property to
vsp1_du_atomic_config allows the R-Car DU driver to pass the
premultiplied alpha plane.

Signed-off-by: Takanari Hayama <taki@igel.co.jp>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>