attach(In reply to comment #5)
> Created attachment 81945 [details] [review] [review]
> Debugging patch
> 
> Please try the attached patch, it shouldn't fix the issue but instead
> outputs some debug messages ("IH: CP....") into dmesg.
> 
> If you don't see those messages try to but the system under some graphics
> load, for example start glxgears ten times simultaneously or something like
> that.

Yes, that is what happened:

[  334.326551] [drm:evergreen_irq_process] *ERROR* IH: CP EOP
[  334.326700] [drm:evergreen_irq_process] *ERROR* IH: CP EOP
[  334.326922] [drm:evergreen_irq_process] *ERROR* IH: CP EOP
[  334.327071] [drm:evergreen_irq_process] *ERROR* IH: CP EOP
[  334.327190] [drm:evergreen_irq_process] *ERROR* IH: CP EOP

Attaching full dmesg as well.