logind: set session->started before seat_read_active_vt() call

logind: set session->started before seat_read_active_vt() call

session_start() calls seat_read_active_vt() before setting
s->started = true. seat_read_active_vt() can reach
seat_triggered_uevents_done() synchronously via:

  seat_active_vt_changed -> seat_set_active -> seat_trigger_devices
  -> seat_triggered_uevents_done

When seat_trigger_devices() produces no pending uevents,
seat_triggered_uevents_done() runs in the same call stack as
session_start() and tests session->started before the assignment
further down. The check fails, session_device_resume_all() is
skipped, and the compositor never receives DRM master.

Set started before seat_read_active_vt() so the gate sees the
correct value, and document the ordering constraint at the call
site to prevent regression.

Reproducible with greetd plus a Wayland compositor on the same VT
on kernel 6.19+.

Fixes: #41562
Signed-off-by: countgitmick <263313427+countgitmick@users.noreply.github.com>