https://bugs.freedesktop.org/show_bug.cgi?id=109001
--- Comment #12 from wirch.eduard@gmail.com --- Disabling new display code does help indeed. Thanks for the hint. Hopefully the recent patch set for 4.21 (https://www.phoronix.com/scan.php?page=news_item&px=AMDGPU-Fixes-For-Lin...) fixes this problem.