It seems the problem is the dpm clock control for the primary video card.  Setting power_dpm_force_performance_level to manual and then setting pp_dpm_sclk has no effect (it stays at 0).
https://www.phoronix.com/forums/forum/linux-graphics-x-org-drivers/open-source-amd-linux/871595-amd-delivers-overdrive-overclocking-support-for-amdgpu-drm-driver/page4

I think something with Xorg may be interfering with the clock settings, so I'm going to try running X off the on-board video.