As long as the jockey is still attached to the horse at the finish, they aren't disqualified, even if they're just hanging onto its neck.
The position that jockeys ride in makes them less wind resistant. It also puts them directly on the horse's center of gravity, so they can balance best and the horse can run all out comfortably. So there'd be no reason for a jockey to purposely drop their stirrups and sit, and if they did it would probably cause the horse to lose. It has happened when jockeys accidently lost their stirrups, though.