Right now every backend clears output damage from the primary plane when
it repaints. Instead of having this same operation spread across all
the backends, just do it in the core instead.
In the future, we want to remove damage tracking from the primary plane
entirely, and this is a small step in that direction.
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
f07af89f9c