WARNING in idr_destroy

[syzbot]

syzbot has bisected this bug to:

commit 5b3709793d151e6e12eb6a38a5da3f7fc2923d3a
Author: Thomas Zimmermann <tzimmermann at suse.de>
Date:   Wed May 8 08:26:19 2019 +0000

    drm/ast: Convert AST driver to |struct drm_gem_vram_object|

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15a66fb5e00000
start commit:   63623fd4 Merge tag 'for-linus' of git://git.kernel.org/pub..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=17a66fb5e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=13a66fb5e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5d2e033af114153f
dashboard link: https://syzkaller.appspot.com/bug?extid=05835159fe322770fe3d
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14e978e3e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10b1a819e00000

Reported-by: syzbot+05835159fe322770fe3d at syzkaller.appspotmail.com
Fixes: 5b3709793d15 ("drm/ast: Convert AST driver to |struct drm_gem_vram_object|")

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