Add a helper to set the background color during vpdma transfer.
This is needed when VPDMA is generating 32 bits RGB format
to have the Alpha channel set to an appropriate value.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>