output: Set backend surface only after creation
output: Set backend surface only after creation

This should avoid https://github.com/SirCmpwn/sway/issues/847 happening
if creation is prevented.

And should also fix #190

Cloudef/wlc,yohanesu75/wlc,vially/wlc,Enerccio/ewlc,vially/wlc,yohanesu75/wlc,Enerccio/ewlc,Cloudef/wlc