“Syd Barrett,” Roddy Bogawa writes in the notes accompanying a new documentary, “just happened to become one of the most famous cult icons in music.” This raises a question: What is the criterion for such a designation? If “Have You Got It Yet? The Story of Syd Barrett and Pink Floyd,” the film Mr. Bogawa co-directed with the late Storm Thorgerson, is an indication, the answer is neither simple nor happy.