drm/hisilicon: Enable the shadowfb for hibmc

set the prefer_shadow as 1,so we use the shadowfb to acceleration.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Gong junjie <gongjunjie2@huawei.com>
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1578712146-16327-1-git-send-email-tiantao6@hisilicon.com