On Ubuntu Bionic with kernels 4.15 and above, RX480 Displayport outputs don't work (dmesg reports "not connected")

Submitted by Pixie

Assigned to Default DRI bug account

Link to original bug (#106718)

Description

Anything from at least 4.15 up to 4.17-rc1 under Bionic makes the Displayport connections fail to even detect a display, while anything 4.14.x works and detects it perfectly.

Is the PCI ROM error related? Is this a duplicate? I can provide xorg logs later today.