This issue can be reproduced on Linux 4.18.5 on Asus X570VD (AMD Ryzen 7 with
Radeon Vega Mobile Gfx) which includes the above patch.

It can also be reproduced on the latest 4.15 kernel in Ubuntu 18.04.

In both cases the glitch is quite jarring, lots of white flashing up in the
middle of the screen.

During ubuntu boot:
https://youtu.be/cOYumE38Xac

During manual modprobe:
https://youtu.be/I1a6C4mF6pA

I installed AMDGPU-PRO 18.20 All-Open version and the problem remains.

Please let me know how we can assist with next steps.