The fall TV season has begun in earnest, and we're already prepared to name the first breakout star. His name? Mark-Paul Gosselaar. (No, you're not reading an issue from 1990.) The onetime Saved by the Bell cutup headlines the new TNT law drama Raising the Bar, which drew 7.7 million viewers on Sept. 1. It was the biggest series premiere ever for an ad-supported cable network and it more than doubled the audience for the highly touted return of Gossip Girl on the same night. That's a pretty substantial feat, especially since critics shrugged off the show and no one predicted any pent-up Gosselaarian desire from TNT viewers. ''Mark-Paul was a definite draw,'' crows Steve Koonin, Turner Entertainment Networks' president. ''I can't tell you the number of e-mails we got about Zack Morris.'' Bar surely benefited from airing after cable's top drama, The Closer and perhaps the uproar over the most controversial new 'do (or don't?) since Keri Russell's Felicity cut her locks almost a decade ago. ''Mark-Paul's hair was a calculated marketing draw,'' adds Koonin, ''because it got people talking.'' Game on, Mario Lopez!