Oh, I see. I wasn't aware of the change. I wondered in the beginning if things
were different because I knew the explicit dc and dpm settings were a temporary
work around, but unfortunately I didn't come across any explicit notices about
whether or not they were still used.

In any case, I'd tried both setting amdgpu.dpm=0 and omitting it completely and
that's when I discovered I could boot without it (if I remember correctly I
believe it wouldn't boot with dpm=0 either). However the problem is that my
computer doesn't resume from suspend with any kernels from 4.19.x on so I
assumed display power management wasn't enabled. But if I understand you
correctly it seems it's enabled but not working with resume for some reason.

And by the way, thank you for working on this problem so diligently with me.
I've had a lot of problems with my beloved R9 390 and it's nice to see it
getting the attention it deserves! :)