drm/rockchip: rgb: embed drm_encoder into rockchip_encoder

Commit 540b8f27 ("drm/rockchip: Embed drm_encoder into
rockchip_decoder") provides the means to pass the endpoint ID to the
VOP2 driver, which sets the interface MUX accordingly. However, this
step has not yet been carried out for the RGB output block. Embed the
drm_encoder structure into the rockchip_encoder structure and set the
endpoint ID correctly.

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/20230124054706.3921383-3-michael.riesch@wolfvision.net