Now that Mel Gibson has conquered the box office, he aims to take his passion to the small screen. Gibson's people have quietly been in talks with the broadcast nets about buying the rights to The Passion of the Christ. None of them are willing to make a deal yetmostly because of the cross they'd bear: Sources say Gibson's reps are asking the nets to find a single advertiser to sponsor the movie without commercial breaks -- like NBC's 1997 presentation of Schindler's List. But that may not be easy. ''There would be political pressure,'' says one ABC insider, adding that advertisers might balk at the film's extreme violence. Still, says an NBC suit, ''it would get huge ratings.'' From his lips to Mel's ears.