Comment # 19 on bug 1134805 from Mindaugas Baranauskas

by default "AccelMethod" is set to "EXA".

I followed https://en.opensuse.org/SDB:Radeon recommendation and set:

Section "Device"
  Identifier "Default Device"
  Driver "modesetting"
  Option "AccelMethod" "XAA"
  Option "EnablePageFlip" "on"
EndSection

after reboot, I don't see graphical artifacts.