perf mmap: Move perf_mmap and methods to separate mmap.[ch] files

perf mmap: Move perf_mmap and methods to separate mmap.[ch] files

To better organize the sources, and we may end up even using it
directly, without evlists and evsels.

Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>