drm/rockchip: vop: group vop registers

Grouping the vop registers facilitates make register
definition clearer, and also is useful for different vop
reuse the same group register.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1501221986-29722-1-git-send-email-mark.yao@rock-chips.com