drm/rockchip: rgb: add stub functions when rgb encoder is disabled

The newly added internal rgb encoder for Rockchip vops is missing
stubs for the case that the rgb output part is not enabled in the
kernel config. So add these.

Fixes: 1f0f0151 (drm/rockchip: Add support for Rockchip Soc RGB output interface)
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
[seanpaul fixed up checkpatch nits]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905191302.26023-1-heiko@sntech.de