In case anyone is interested, I've hacked together a GNOME extension that adds
a status icon showing the current DPM setting and a popup menu to easily
switch: https://github.com/JuBan1/radeon-dpm-control.git

It only handles three combinations of settings: low/battery, auto/balanced,
high/performance. I don't think the others are particularly useful.

It makes use of Thomas DEBESSE's dpm-query script a few comments earlier.