libweston/backends: Move damage flush into backends

Currently we flush damage for the "primary plane" every repaint, but this
is folly.

The drm backend may skip rendering entirely if using an all-planes
composition. This could leave the renderer plane in a messy state if a
surface on an overlay plane disappears.

Instead, let the backends flush the primary plane damage when they know
they need to render.

Fixes #864

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>