kms: Only emit resources-changed signal if we recorded a change
Since we cache already all the KMS parameters we care about let's check at each device update if anything has really changed and only in such case emit a resources-changed signal.
In this way we can also filter out the DRM parameters that when changed don't require a full monitors rebuild.
Examples are the gamma settings or the privacy screen parameters, that emits an udev "hotplug" event when changed, but we want to register those only when we handle the changed property.
This is the prerequisite of !1952