You can force the clocks low or high by:
echo low > power_dpm_force_performance_level
or
echo high > power_dpm_force_performance_level
setting it to auto will restore the automatic behavior:
echo auto > power_dpm_force_performance_level

The behavior will depend on the workload.  If the workload is really bursty, it
may cause the clocks to ramp up and down if there are sufficiently long idle
periods between workloads.  You can manually adjust the heuristics by selecting
the custom profile and tweaking each parameter.  See the documentation here:
https://dri.freedesktop.org/docs/drm/gpu/amdgpu.html#gpu-power-thermal-controls-and-monitoring