drm/hisilicon/hibmc: add gamma_set function

Add gamma_set function, and we can also use it to adjust the brightness
of the display.

Signed-off-by: Zhihui Chen <chenzhihui4@huawei.com>
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Acked-by: Xinliang Liu <xinliang.liu@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191223074910.1030-1-chenzhihui4@huawei.com