For a biographical drama to work, it’s not required that the central actor do a pinpoint uncanny, so-close-to- the-real-thing-you-might-think-it’s-the-real-thing impersonation of the person he’s playing. But Peter Farrelly’s “I Play Rocky” may be a special case. It’s a movie about the making of “Rocky” — about how Sylvester Stallone, then a two-bit actor in his