The presidential election is more than a year away, but Republican front-runner George Bush can count on one old friend to vote for him: master vibist Lionel Hampton. Hampton--whose band will play Sunday at the Playboy Jazz Festival here--is a longtime Republican and intimate of both Ronald Reagan and Richard Nixon. "I know George Bush from when he was a teen-ager," the 78-year-old musician said. "I used to go over from New York and play rallies for his father (the late Sen. Prescott Bush, R-Conn.). "And I met George Bush 'cause George Bush was the one that used to get the Coca-Cola for us."