[syzbot] [dri?] WARNING in vkms_get_vblank_timestamp (2)

[syzbot]

syzbot has bisected this issue to:

commit ea40d7857d5250e5400f38c69ef9e17321e9c4a2
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Fri Oct 9 23:21:56 2020 +0000

    drm/vkms: fbdev emulation support

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1282dbffe80000
start commit:   6764c317b6bb Merge tag 'scsi-fixes' of git://git.kernel.or..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=1182dbffe80000
console output: https://syzkaller.appspot.com/x/log.txt?x=1682dbffe80000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2c0ac5dfae6ecc58
dashboard link: https://syzkaller.appspot.com/bug?extid=93bd128a383695391534
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12067e60180000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=102774b7e80000

Reported-by: syzbot+93bd128a383695391534 at syzkaller.appspotmail.com
Fixes: ea40d7857d52 ("drm/vkms: fbdev emulation support")

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