perf top: Use all the lines in the screen

By querying the current number of rows, if the user specifies
the number of entries, use that instead. If the user uses the
'e' command to change the number of lines 0 will mean do it
automatically, any other number disables the auto resizing.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1258407027-384-2-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>