PRINCE OR PAUPER: Can Prince's just-opened new film, "Graffiti Bridge," possibly be a bigger dog than his 1986 bomb "Under the Cherry Moon"? You have to figure that Warner Bros. is worried about the movie's critical reception, since the studio refused to screen it for critics before its release. Why the low profile? "Our marketing position is that we think 'Graffiti Bridge' is an audience film, not a critics' film," said a Warners spokesman. "So we want the audience to discover it for themselves." . . . Meanwhile, the troubled LA Gear sportswear firm is in hot water with Def Leppard, thanks to its current "Unstoppable" TV commercial (starring 49ers quarterback Joe Montana), which uses a lyric excerpt from the rock band's 1989 hit single "Armageddon." The only problem: Def Leppard never authorized the song's use. "It's really abhorrent and we plan to make a big stink about this," said Leppard co-manager Cliff Burnstein. "We've never licensed any Leppard track for anything because the band doesn't want their music endorsing someone else's product." An LA Gear spokeswoman said, "We are looking into it."