My case is completely different. I just tried setting dpm "state" to
"performance" and "level" to "high" while in Plasma 5 session, and I got no
problem at all. I didn't use any special kernel parameter, so I have dpm on. I
could even play some games without crash, but still with some stutter. In fact,
"dmesg" shows nothing about radeon or dpm failing, I guess because it's not
regulating voltage and frequency (which is what seems to make it crash).

So in my case it seems I can switch profiles while in active X session without
any problem. I have kernel 4.2.5, latest mesa, llvm, firmware and libdrm.