perf tools: Support event grouping in hpp ->sort()

Move logic of hist_entry__sort_on_period to __hpp__sort() in order to
support event group report.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1400480762-22852-5-git-send-email-namhyung@kernel.org


Signed-off-by: Jiri Olsa <jolsa@kernel.org>