display: Fix display enabled toggle button visibility
Decide the visibility of the display enable/disable toggle button by n_usable_outputs
instead of n_outputs
.
This fixes an issue where the enable/disable toggle button for a display would be visible when only a single usable display is connected. An example of this is a laptop with a closed lid and a single external monitor.
Closes #2303 (closed).
Edited by Timotej Šulík