This cache allows us to easily ensure that we have a unique anv_bo for
each gem handle.  We'll need this in order to support multiple-import of
memory objects and semaphores.

v2 (Jason Ekstrand):
 - Reject BO imports if the size doesn't match the prime fd size as
   reported by lseek().

Reviewed-by: Chad Versace <chadversary@chromium.org>