syzbot has bisected this bug to:

commit 979c11ef39cee79d6f556091a357890962be2580
Author: Ayan Kumar Halder [off-list ref]
Date:   Tue Jul 17 17:13:46 2018 +0000

     drm/sun4i: Substitute sun4i_backend_format_is_yuv() with format->is_yuv

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?xd2f97ee00000
start commit:   596cf45c Merge branch 'akpm' (patches from Andrew)
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?xd2f97ee00000
kernel config:  https://syzkaller.appspot.com/x/.config?x}8ab2e0e09c2a82
dashboard link: https://syzkaller.appspot.com/bug?extid¤ae1442ccc637162dc1
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x273edae00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?xe7677ae00000

Reported-by: syzbot+a4ae1442ccc637162dc1@syzkaller.appspotmail.com
Fixes: 979c11ef39ce ("drm/sun4i: Substitute sun4i_backend_format_is_yuv()  
with format->is_yuv")

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