First time flying DL internationally (route is LAS-SEA-NRT) and not checking bags - can my passport be scanned at the gate (and is that done at LAS or SEA?), or do I need to go to the main check-in area at LAS before going through TSA?

Asking because the garage entrance to the terminal at LAS is right by the TSA entry point for the D gates (which is where DL is), so if I can avoid going all the way to the main check-in area that would save a good amount of time. Thanks