commit 9ac0934b upstream.

The size here comes from the user via the ioctl, it is a number between
1-u32max so the addition here could overflow on 32 bit systems.

Fixes: f453ba04 ('DRM: add mode setting support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>