rv770_restrict_performance_levels_before_switch failed

[Andy Furniss]

Built drm next yesterday was running drm fixes from a couple of weeks 
ago previously, card is rv790.

Have powered up twice since and both times seen logged -

[drm:rv770_dpm_set_power_state] *ERROR* 
rv770_restrict_performance_levels_before_switch failed

Today was 79 sec after boot, though I don't startx automatically so it 
may have been soon after that.

Clock switching does seem to be working OK.