!7979 drm/radeon: fix a possible null pointer dereference

Merge Pull Request from: @ci-robot 
 
PR sync from: Liu Chuang <liuchuang40@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/NKMJ3YPJIBIIEVZJB5O7N676ZYYVVS5R/ 
 
https://gitee.com/src-openeuler/kernel/issues/I9R4KL 
 
Link:https://gitee.com/openeuler/kernel/pulls/7979

 

Reviewed-by: Xu Kuohai <xukuohai@huawei.com>
Reviewed-by: Liu YongQiang <liuyongqiang13@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>