FBI agents have arrested the keyboardist from Detroit rock band Mac Saturn on child pornography charges hours ahead of the group’s headlining show Friday at The Fillmore Detroit, an event billed as a coming-out-party for an act with a growing fanbase. Farmington resident Evan Daniel Mercer, 30, was jailed until Monday at the earliest after a brief hearing in federal court Friday in Detroit, less than a mile from the concert venue. He is facing several charges, including enticement of a minor and producing and/or attempting to produce child pornography. If convicted, he would face a mandatory-m…