perf hists: Add a couple of hists stat helper functions

Add hists__{reset,inc}_[filter_]stats() functions to cleanup accesses
to hist stats (for output).  Note that number of samples in the stat
is not handled here since it belongs to the input stage.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1398327843-31845-5-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>