drm: rockchip: add missing registers for RK3188

Add dither_up, dsp_lut_en and data_blank registers to enable their
respective functionality for RK3188's VOP.
While at that also fix .dsp_blank register which is (only) set with
BIT24 (same as RK3066)

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210528130554.72191-3-knaerzche@gmail.com