!560 [sync] PR-539: LoongArch: fix compile warnning of drm/loongson driver

Merge Pull Request from: @openeuler-sync-bot 
 

Origin pull request: 
https://gitee.com/openeuler/kernel/pulls/539 
 
fix compile warnning by remove unused function and variable. 
 
Link:https://gitee.com/openeuler/kernel/pulls/560

 

Reviewed-by: Jialin Zhang <zhangjialin11@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>