drm: zte: move CSC register definitions into a common header

The CSC (Color Space Conversion) block in VOU is used by not only
Graphic Layer (plane) but also channel (CRTC) module.  Let's move
its register definitions into a common header, so that CRTC driver can
include it when needed.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1491490870-6330-3-git-send-email-shawnguo@kernel.org