perf tools: Bump default sample freq to 4 kHz

Quoting Ingo:

"While at it I'd also suggest increasing the default sampling frequency,
from 1000 Hz per CPU to at least 4Khz auto-freq or so - this should work
well all across the board I think. CPUs are getting faster and command/app
run times are getting shorter, 1Khz is a bit low IMO."

Requested-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-2jafa6mkrufyekny9ei59lpu@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>