Comment # 30 on bug 110865 from Alex Deucher

(In reply to Martin from comment #29)
> @Alex Deucher
> Is there a fix for the graphical glitches I experience? 
> They seem to be similar to the glitches I get when I enable overclocking
> with amdgpu.ppfeaturemask=0xffffffff

It would appear that the monitors don't actually quite sync up in your case
otherwise you wouldn't see the flicker.