GPU RX580 Nitro+ over DisplayPort to a single AOC 75hz monitor. Issue is on
last kernels 4.18 4.19 but not in 4.14.

Screen starts flickering until reboot if I sleep/wake or if I manually switch
the monitor off/on.

Tried to turn amdgpu.dc=0, screen then flickers everywhere anytime unless I
manually change frequency from 75hz to 60hz. That only solves it inside KDE
though.

Checking gpu related logs shows this error, but couldn't find anything else:
"Invalid PCI ROM header signature: expecting 0xaa55, got 0xffff"