Bill Cosby answers the phone: "I'm here!" It's 7 a.m., he's in Los Angeles, and his greeting feels like a pronouncement, at once comic and serious. Yes, Cosby is here, and he is about to hold court — about jazz, his passion since he was a kid in the North Philadelphia housing projects, dancing to Clarence "Bull Moose" Jackson records and dreaming of becoming a drummer. That didn't work out, but, as with the best jazz musicians, Cosby — comedian, TV and film star, educator, producer — is