In Software & Updates | Additional Drivers (where I select the AMD graphics drivers) I see a new entry:
Unknown:Unknown This device is not working.
With two options:
[ ] Using Processor microcode firmware for AMD CPUs from amd64-microcode (proprietary) [*] Do not use the device
Is the "device is not working" line saying don't enable this option because it doesn't work anyway or maybe you should enable it because the (cpu) device is not working correctly?