elementaryOS 0.4.1, GTX 1080
Broken: 2.79 e32c1bd)
Short description of error
The CUDA devices are not reported properly by C.user_preferences.addons['cycles'].preferences.devices. It labels the wrong card as display. I guess it happened when GPU+CPU was introduced recently.
Exact steps for others to reproduce the error
Probably you need a multiGPU setup. If you compare the settings in user-preferences with what the python console is reporting, there seems to be a mismatch.
The display in my computer is only connected to the GTX980 Ti, correctly reported by user-prefs, but not py Python.