Some common minimums for zero clearance units are 6" to combustibles (wood surrounds etc.) to the sides and at least 1 foot obove the opening before a mantel shelf. It is probably pretty similar for masonry fireplaces. Your local building inspector or fire marshal/chief may have his own specific requirements. One way to be safe is to use cement board instead of drywall directly over and around the brick face. You can nail it to the studs just like the drywall and use thinset mortar to attach to the bricks (after you remove or rough up some of the brick paint). If the cement board is flush to the surrounding drywall then just tape and mud the joint just like any other. You will have to skim the cement board to fill in any irregularities. You can either just leave it like that, add some tile to taste, use a wood surround/mantel, or whatever.