perf stat: Use xyarray dimensions to iterate fds

Now that the xyarray stores the dimensions we can use those
to iterate over the FDs for a evsel.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20171006020029.13339-1-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>