wayland: Refactor output initialization code

wayland: Refactor output initialization code

Summary:
The code to initialize outputs has duplicate stanzas for updating the
data members of the output structure.  This set of refactoring steps
shuffles things around to eliminate the redundant code.  I think this
also makes the code easier to follow too.

This also breaks out a new internal routine for looking up outputs by
id.  I suspect this will have general usefulness later.

Reviewers: devilhorns, cedric, zmike

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D2077