perf top: Fix number of samples displayed

In recent versions of perf top, pressing the 'e' key to change the
number of displayed samples had no effect.

The number of samples was still dictated by the size of the terminal
(stdio mode). That was quite annoying because typically only the first
dozen samples really matter.

This patch fixes this.

Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120130105037.GA5160@quad
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>