That line appears fairly early in the story. In this particular usage of "fixed," it means stationary and secured. A table on a ship might be "fixed" to the floor to prevent it from sliding back and forth as the ship rolls on the waves. In "King of the Bingo Game," the narrator is thinking about how amazing it is that the movie projector's beam of light always hits the screen at exactly the right distance and size. He knows why this is. He knows that the projector is in a fixed position in the projector room, and he knows that the screen is in a fixed position a certain distance away from the projector. The narrator thinks to himself, "Everything was fixed." He knows that in addition to the projector and screen, the seats are fixed to the floor. The line also is slightly referencing the film that he is watching. The narrator admits that he has seen the film before, so nothing in it surprises him. He knows that nothing will be changed. The events of the movie that he is watching are fixed.