perf_counter: tools: report: Rework histogram code

In preparation for configurable sorting, rework the histgram code a bit.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20090527182100.796410098@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>