Mutter recently gained the ability to deal with multiple GPUs
rendering at different displays. These GPUs would have a display
connected to them, and Mutter was adapted in order to be aware
of different GPUs and their outputs.

However, one specific edge case appeared: PRIME systems. PRIME
systems have two GPUs:

 * The integrated GPU (iGPU), usually Intel, which has connectors
   and deals with the routine load.

 * The dedicated GPU (dGPU), usually AMD or NVidia, which has no
   connectors at all and are there just to aid heavy loads.

On those systems, the dGPU is aggressively put to sleep by the
kernel to avoid energy waste. Waking it up is a costly operation.

With Mutter's adaptation to deal with multiple GPUs, Mutter began
wakening the dGPU every time some rendering had to be done. This
was causing stuttering every time the dGPU was put to sleep, and
Mutter asked it to wake up again.

To fix this situation, this commit ignores GPUs with no connectors
attached.

Issue: #77