perf session: Change perf_session post processing functions to take histogram tree

Now that report can store historgrams for multiple events we
need to be able to do the post processing work for each
histogram. This patch changes the post processing functions so
that they can be called individually for each event's histogram.

Signed-off-by: Eric B Munson <ebmunson@us.ibm.com>
[ Guarantee bisectabilty by fixing up builtin-report.c ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1267804269-22660-5-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>