drm: zte: add VGA driver support

It adds VGA driver support, which needs to configure corresponding VOU
interface in RGB_888 format, and thus the following changes are needed
on zx_vou.

 - Rename the CSC block of Graphic Layer a bit to make it more specific,
   and add CSC of Channel to support RGB output.
 - Bypass Dither block for RGB output.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491910226-7831-1-git-send-email-shawnguo@kernel.org