It seems I also spoke to soon, because I tried again with mclk disabled but
this time with the test method where I forced the low and high power levels by
using  /sys/class/drm/card0/device/power_dpm_force_performance_level and this
time I got a lockup. 

So either it was just chance that I didn't hit the lockup with mclk disabled
the first time and/or my second test method have a higher chance of triggering
the lockup.

I'm currently running a test with dynamic_ss disabled.