damage_ring: Fix forwarding damage

Consider this situation: We are running the DRM backend that may want 3 or 2 buffers in rotation. Maybe DRM gets a little bit backed up and asks for a third one. However, normally it only needs to be double buffered. While in the double buffered state, we would basically forget about accumulated damage during this time for the third buffer in the queue and so next time someone asks for it, it will report incorrect damage.

I'm not so sure if the "simplicity" this solution affords is worth it anymore as it's difficult code to get right...