Put fillets, skin sides down, in a lightly oiled large shallow baking pan. Stir together ginger, garlic, lime juice, oil, salt, chili powder, and turmeric in a small bowl until combined well, then rub onto mackerel flesh and marinate 10 minutes. Preheat broiler. Broil mackerel 5 to 6 inches from heat, without turning, until cooked through and lightly browned, 7 to 8 minutes. Transfer to plates with 2 spatulas and top with cilantro.