compositor-drm: Add shutting_down flag

Does what it says on the box: is true when the compositor is shutting
down.

When we begin to use universal planes, we need divergent destruction
paths. With universal planes, the drm_planes are created at backend
initialisation time, and destroyed with the backend. However, without
universal planes, we create per-output drm_planes to hold the
primary/scanout and cursor planes, whose lifetime is tied to the output.

We will use the new shutting_down flag to determine if output
destruction is hot-unplug or compositor shutdown, and make a decision on
whether or not to destroy the special planes.

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

libweston/compositor-drm.c

@@ -184,6 +184,8 @@ struct drm_backend {
 	int32_t cursor_height;
 
 	uint32_t pageflip_timeout;
+
+	bool shutting_down;
 };
 
 struct drm_mode {
@@ -3605,6 +3607,8 @@ drm_destroy(struct weston_compositor *ec)
 	wl_event_source_remove(b->udev_drm_source);
 	wl_event_source_remove(b->drm_source);
 
+	b->shutting_down = true;
+
 	destroy_sprites(b);
 
 	weston_compositor_shutdown(ec);