perf report: Change default callchain parameters

The default callchain parameters are set to use the flat mode and never
filter any overhead threshold of backtrace.

But flat mode is boring compared to graph mode.
Also the number of callchains may be very high if none is
filtered.

Let's change this to set the graph view and a minimum overhead of 0.5%
as default parameters.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Anton Blanchard <anton@samba.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1246772361-9960-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>