perf hists: Add more helpers for hist entry stat

perf hists: Add more helpers for hist entry stat

Add and use he_stat__add_{period,stat} for calculating hist entry's
stat.  It will be used for accumulated stats later as well.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Arun Sharma <asharma@fb.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1349354994-17853-10-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>