perf report: Treat an argument as a symbol filter

As Ingo requested, it'd be better off treating first (and the only)
argument as a symbol filter, so that user doesn't need to input the
symbol on the dialog window on TUI.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1331887855-874-5-git-send-email-namhyung.kim@lge.com


Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>