main: Call RegisterDisplay once UI has initialized

main: Call RegisterDisplay once UI has initialized

RegisterDisplay is called to signal that graphics have settled,
which allows GDM to terminate pending greeters and plymouth.

Previously, RegisterSession was used for both recording the login
and terminating greeters. This caused a race condition where the
new user shell was setting up while the greeter was tearing down,
leading to configuration conflicts during the asynchronous handoff.

By splitting into RegisterSession (called early to record login) and
RegisterDisplay (called after graphics settle), the greeter teardown
is delayed until the new session has completed its initial modeset.

Fixes: 32a01330 ("main: Register session with GDM on startup")
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4057>