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)