perf hists: Make hpp setup function generic

Now that we have the 'equal' method implemented for hpp format entries
we can ease up the logic in the following functions and make them
generic wrt comparing format entries:

  perf_hpp__setup_output_field
  perf_hpp__append_sort_keys

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/1453109064-1026-8-git-send-email-jolsa@kernel.org


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