!509 Loongson: fix 7a2000 gpu driver can not work

Merge Pull Request from: @Hongchen_Zhang 
 
After apply this patch,we can boot 3a5000+7a2000 machine normally. 
 
Link:https://gitee.com/openeuler/kernel/pulls/509

 

Reviewed-by: Guo Dongtai <guodongtai@kylinos.cn>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>