I just hit this as well with 4.19 on Fedora and a R9 390X - Grub shows fine,
then no video output after that (monitor goes into power save), and boot
doesn't seem to continue (no disk activity etc.)

I removed amdgpu.dpm=1 from my kernel params and was able to boot with 4.19.x -
I noticed that everyone who mentions kernel params on this bug has this param
present too - try without it?

This is a regression vs. 4.18.x - with that kernel series I was able to boot
with amdgpu.dpm=1 without issue.