drm/rockchip: vop2: Support setting custom background color

drm/rockchip: vop2: Support setting custom background color

The Rockchip VOP2 display controller allows configuring the background
color of each video output port.

Since a previous patch introduced the BACKGROUND_COLOR CRTC property,
which defaults to solid black, make use of it when programming the
hardware.

Note the maximum precision allowed by the display controller is 10bpc,
while the alpha component is not supported, hence ignored.

Tested-by: Diederik de Haas <diederik@cknow-tech.com>
Reviewed-by: Andy Yan <andyshrk@163.com>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://patch.msgid.link/20260303-rk3588-bgcolor-v8-4-fee377037ad1@collabora.com
Signed-off-by: Daniel Stone <daniels@collabora.com>