!8465 drm/amd/display: Avoid NULL dereference of timing generator

Merge Pull Request from: @ci-robot 
 
PR sync from: Wenyu Huang <huangwenyu5@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/Z27JIOPVSOLDD452G5MPIFARYMG7ZTCJ/ 
 
https://gitee.com/src-openeuler/kernel/issues/I9R4M5 
 
Link:https://gitee.com/openeuler/kernel/pulls/8465

 

Reviewed-by: Zucheng Zheng <zhengzucheng@huawei.com>
Reviewed-by: Liu YongQiang <liuyongqiang13@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>