[syzbot] general protection fault in drm_client_buffer_vunmap

[syzbot]

syzbot suspects this issue was fixed by commit:

commit 874a52f9b693ed8bf7a92b3592a547ce8a684e6f
Author: Tong Zhang <ztong0001 at gmail.com>
Date:   Sun Feb 28 04:46:25 2021 +0000

    drm/fb-helper: only unmap if buffer not null

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10c27b7ed00000
start commit:   c03c21ba Merge tag 'keys-misc-20210126' of git://git.kerne..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=ec4c85e44cc3172e
dashboard link: https://syzkaller.appspot.com/bug?extid=10328e8428a896b65119
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12d95d7ad00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=148da9ccd00000

If the result looks correct, please mark the issue as fixed by replying with:

#syz fix: drm/fb-helper: only unmap if buffer not null

For information about bisection process see: https://goo.gl/tpsmEJ#bisection