Do not re-read output configuration if the outputs didn't change

They would override KScreen in case we were using a dock station that
brings 2 displays.
We'd get:
- udev: event for the first hotplughed screen
- kwin: process all screens properly (both)
- kscreen: would offer the right configuration for such displays
- udev: process the event for the second hotplug udev event
- kwin: restore the configuration
- kscreen: would think this is a conscious decision and embrace it as a
configuration

With this change we are only re-reading the configuration in case the
outputs changed.