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>