Fix hotplugging for VR headsets
When hotplugging VR headsets the non-desktop property does not get updated because we're creating all connectors when the function is first called, even if they're not connected. Then KWin tries to use the headset as a normal screen.
This change simply makes it ignore connectors if they're not connected. I'll probably completele rewrite DrmGpu::updateOutputs sometime soon but this makes it work for now