Send SurfaceInterface::enter events after clients bind to outputs

The following order of events are legal:

Typically order is:
 - server announces a new output
 - client binds to a new output
 - server updates the surface to be on new output

But we can have events occur in the following order:

 - server announces a new output
 - server updates the surface to be on new output
 - client binds to a new output

At which point when we update the surface there is no ID to tell the
client which output the surface is on.

This patch watches for clients binding to output and updates
appropriately.