drm/mediatek: Remove the unneeded result

Return the value drm_mode_config_helper_suspend() directly instead of
 storing it in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220825072335.228921-1-ye.xingchen@zte.com.cn/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>