Fix binding issue with performance overlay

* when user disables the overlay by clicking on it there the binding as it is implemented
  doesn't work and the values from the overlay and the service will be out of sync causing
  us not being able to activate the overlay again.

Signed-off-by: Simon Busch <morphis@gravedo.de>
Merge pull request #136 from webOS-ports/performance

Fix binding issue with performance overlay