https://bugs.freedesktop.org/show_bug.cgi?id=94874
--- Comment #7 from Michel Dänzer michel@daenzer.net --- Please attach the full dmesg.
Looks like the fundamental issue may be the kernel driver failing to initialize hardware acceleration after turning the GPU back on, related to UVD/VCE. Does radeon.runpm=0 on the kernel command line avoid the problem?