drm/rockchip: vop: Support dithering to RGB666

Splits out the dither register bits and introduces
the same config enumerations as in the rockchip kernel tree.
Tested to fix the banding on my ASUS C201.

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318154412.26994-1-urjaman@gmail.com