kms/crtc: Don't report predicted gamma changes

We started to report resource changes using prediction when an update had been successfully committed. While at it, gamma changes were reported too, but this was problematic, as gsd-color will listen for the MonitorsChanged D-Bus signal and naively set the gamma again, even if it didn't change. There aren't currently any actual use cases for being told when gamma changes from a prediction, so just ignore it and just report privacy screen changes.

This avoids a feedback loop between mutter and gsd-color.

Fixes: 81b28a1d