https://bugs.freedesktop.org/show_bug.cgi?id=82201
--- Comment #28 from Chernovsky Oleg adonai@xaker.ru --- Did you try to "echo auto > /sys/class/drm/card0/device/power_dpm_force_performance_level"?
It may be related to bug #79806 (Performance degradation after resume), that should be fixed by patch I've sent to Alex recently.