The recent patches to amdgpu made those annoying kernel warning messages go
away for me, too. Along with them went some initialization and self-test
messages, such as e.g.

> [drm] GPU post is not needed
> [drm] amdgpu: irq initialized.
> amdgpu: [powerplay] amdgpu: powerplay sw initialized
... which probably (hopefully) were not really relevant, either.