I found out some interesting fact on my system:

If I start in textmode (runlevel 3) with no specific kernel parameter like
'radeon.dpm=0', set power_dm_force_perofrmance_level to 'high' and
power_dpm_state to 'performance' and finally start the gdm service, the system
works without crash. I was even able to play some games.

If I boot in textmode with radeon.dpm=0 and try to set power_profile to 'high',
I get a crash immediately. I cannot even start gdm in this case.