drm/hisilicon/hibmc: fix 'xset dpms force off' fail

both crtc_state->adjusted_mode.hdisplay and
crtc_state->adjusted_mode.vdisplay are 0 when switch dpms off,
return -EINVAL cause switch dpms off fail.

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/20191220023004.2658-1-chenzhihui4@huawei.com