v4l: vsp1: bru: Support premultiplied alpha at the BRU inputs

v4l: vsp1: bru: Support premultiplied alpha at the BRU inputs

Adjust the BRU blending formula to avoid the multiplication by alpha
when the corresponding input format is premultiplied. As this requires
access to the RPFs connected to the BRU inputs from the BRU module,
store pointers to the RPFs in the BRU structure when validating the
pipeline.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
(cherry picked from commit 6418b4d6e1f1d70199a93eeeed948be2229b9c1a)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>