How many T-shirts do you have to sell to become the owner of a $12 million Malibu mansion? You'll have to defer to James Perse for the answer. The California designer, known for his eponymous line of luxurious basics like tees, sweats, and fleece hoodies, is putting his Malibu pile up for sale, furniture included, for the pretty price of $12 million, reports the Wall Street Journal. The "farmhouse" is consists of 6 bedrooms over 7,400 square feet, located on two acres of lane in the…