compositor(-drm): add support for plane damage

Hi All,

With a patch series over dri-devel adding support for damage during plane update (page-flip), this merge request is an implementation in weston. Currently only sending damage for default scanout plane.

Link to drm damage interface patch series - https://lists.freedesktop.org/archives/dri-devel/2018-September/188725.html

Thanks, Deepak