compositor-drm: Turn vblank_pending from bool to refcount

compositor-drm: Turn vblank_pending from bool to refcount

vblank_pending is currently a bool, which is reset on every vblank
requests (i.e. sprite pageflip). This can occur more than once per
frame, so turn it into a callback, so we only fire frame-done when we've
collected all the events.

This fixes unexpected behaviour when multiple views per output have been
promoted to DRM planes.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>