adaptive backlight management broken on vega since 6.12.5

Setting /sys/class/drm/card0-eDP-1/amdgpu/panel_power_savings to any non-zero value when the laptop (a Lenovo T14 gen2 with Ryzen 5850U) is on battery causes the brightness to climb, then flicker, then drop to zero until the laptop is rebooted.

At runtime, setting panel_power_savings=0 via "echo 0 > /sys/class/drm/card0-eDP-1/amdgpu/panel_power_savings" instantly restores the backlight functionality. Setting it back to 1 restarts the buggy behavior.

The issue doesn't seem to be visible right after boot, but requires a suspend/resume cycle on this system (which is s3 sleep here).

This is a regression that started with 6.12.5. Still present in 6.12.6 (from debian unstable).

I've found at least another report of this issue in the wild that matches the above: https://discussion.fedoraproject.org/t/kernel-6-12-5-display-brightness-issue-at-resume/140317/4