Some AMD GPUs Affected By A Nasty Power Regression That Snuck Into Linux 4.18 Stable

A Phoronix reader emailed in that since the recent Linux 4.18.10 stable kernel the power usage on his system has increased by around 50 Watts while idling... Not the overall AC system power draw being 50 Watts, but an increase of roughly that amount on the latest 4.18 stable point releases up to this point. I've now been able to reproduce as well as bisect the cause...