local-display-factory: Ensure greeter sessions on non-primary seats are respawned on termination

Trigger ensuring a greeter session for a non-primary seat when it
terminates and only looking for displays with state of
GDM_DISPLAY_MANAGED when checking for duplicates in
ensure_display_for_seat on non-primary seats.
1 job for wayland-mutliseat-enablement in 3 minutes and 27 seconds (queued for 2 seconds)
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