Add conversion helpers between xinerama indices and output objects

Currently, we implicitly assume that enabled outputs are stored in the
xinerama order, but with ongoing refactorings, it's easy to break that
assumption. Also, we are not going to change the DRM backend so it
stores enabled outputs in the xinerama order.

This change adds xinerama index conversion helpers in order to reduce
the number of potential regressions with output refactorings.