!6578 drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip

Merge Pull Request from: @ci-robot 
 
PR sync from: Xiongfeng Wang <wangxiongfeng2@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/VULVZ7FSBX3M72RIOKS6K4IDN2IT3KRA/ 
 
https://gitee.com/src-openeuler/kernel/issues/I9HK5V 
 
Link:https://gitee.com/openeuler/kernel/pulls/6578

 

Reviewed-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>