perf stat: Introduce struct perf_stat_config

perf stat: Introduce struct perf_stat_config

Moving 'aggr_mode' into new struct. The point is to centralize the base
stat config so it could be used localy together with other stat routines
in other parts of perf code.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1437481927-29538-3-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>