backend-drm: obtain DRM backend from listener in session_notify()

The session_listener is embedded in the DRM backend structure.
Use this to obtain the DRM backend with container_of().

That way the DRM backend will not have to be found from the compositor.
This is trivial now, but in the multi-backend case would entail
iterating over all backends to find the correct one.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>