drm/hisilicon: Add the mode_valid function

add mode_valid function, we can make sure the resolution is valid.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Gong junjie <gongjunjie2@huawei.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1578476501-45807-1-git-send-email-tiantao6@hisilicon.com