backend-drm: handle multiple drm nodes with logind
When using logind launcher, we receive a PauseDevice "gone" message from logind session management for each device we close while looking for KMS devices.
Make logind notify the backend of the device add/remove so that the backend can decide what to do, instead of assuming that if it is a DRM_MAJOR device the session should be (de)activated. The backend can then react to its specific device.
Fixes #251 (closed)
Signed-off-by: Robert Beckett bob.beckett@collabora.com