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

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>