freedesktop, I'm glad to hear removing "amdgpu.dpm=1" allows you to boot 4.19. 
Unfortunately, I've tried that, and it made no difference.  Even if it did
allow booting, that would bring back the 2.5 year old stability bug 91880,
unless that's been fixed in 4.19 to no longer need this and all the other
kernel parameters.  I've also tried github.com/torvalds/linux master with and
without this kernel parameter and the others, and with and without the patch on
bug 108704, and nothing has worked so far.