Simon, could you please test if the issue goes away after adding this to your /etc/X11/xorg.conf:
Section "Device" ... Option "ForceEnablePipeA" "true" EndSection
Laura, please see bug #138256 and file a new bug following the instructions in the [Update] section of that bug's description, and I can add a quirk for your hardware too.
Simon, could you please test if the issue goes away after adding this to your /etc/X11/xorg.conf:
Section "Device"
...
Option "ForceEnablePipeA" "true"
EndSection
Laura, please see bug #138256 and file a new bug following the instructions in the [Update] section of that bug's description, and I can add a quirk for your hardware too.