output: handle outputs that are enabled but not yet added

output: handle outputs that are enabled but not yet added

output_config_apply() currently assumes that any enabled outputs have
already been added to the wlr_output_layout. However, at startup,
wlroots will report that outputs are enabled if they were previously in
use by e.g. the Linux console - but they are not yet in the layout.

output_config_apply() never encountered this case before now because the
initial modeset was performed by another code path, but now with the new
autoEnableOutputs option (next commit), it needs to be able to handle
the initial modeset as well.