drm/rockchip: vop2: add support for the rgb output block

The Rockchip VOP2 features an internal RGB output block, which can be
attached any video port of the VOP2. Add support for this output block.

Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230124054706.3921383-6-michael.riesch@wolfvision.net