https://bugs.freedesktop.org/show_bug.cgi?id=89015
--- Comment #14 from Arek Ruśniak arek.rusi@gmail.com --- Is there any way to switch dri mode? As far i know xorg1.17+modesseting just uses dri3. There is no log with dmesg/xorg/systemd when dri3 is used. So answer for your question is: so probably yes, but i don't know for sure.
DDX from https://github.com/iXit/xf86-video-ati/ has the same symptoms like modeseting but looks worse a lot (this is why i never like it)