drm/hisilicon/hibmc: Add support for VDAC

VDAC(Video Digital-to-Analog converter) converts the RGB diaital data
stream from DE to VGA analog signals.

Signed-off-by: Rongrong Zou <zourongrong@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Xinliang Liu <xinliang.liu@linaro.org>
Acked-by: Sean Paul <seanpaul@chromium.org>