login: mark the Display property as "emits change"

login: mark the Display property as "emits change"

With the introduction of SetDisplay() method in 4885d7490b2 the Display
property emits a "properties changed" message every time the display is
changed using this method, so mark it appropriately.

Caught by systemd/systemd#30029:

systemd-logind[1366]: Got message type=method_call sender=:1.165 destination=org.freedesktop.login1 path=/org/freedesktop/login1/session/_310 interface=org.freedesktop.login1.Session member=SetDisplay  cookie=8 reply_cookie=0 signature=s error-name=n/a error-message=n/a
systemd-logind[1366]: Assertion 'v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE || v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION' failed at src/libsystemd/sd-bus/bus-objects.c:2141, function emit_properties_changed_on_interface(). Ignoring.

Follow-up to 4885d7490b2.