I agree with Trevor. You don't have to have a main breaker in your garage panel itself. But the building will have to have a means for disconnecting power to everything inside.
However, there are cases in which a main breaker in the garage panel will not fulfill the requirement for a building disconnect. The disconnecting means has to be "at a readily accessible location nearest the point of entrance of the conductors." 225.32. Some jurisdictions, and the State of Washington is among them, give a specific limit to the distance from the point of entrance to the location of the disconnecting means. Our limit is 15 feet. So if the garage panel is in the middle of the garage, it may take some extra effort to be able to take credit for the panel's main breaker as being the building's disconnecting means.