perf tools: Introduce reset_output_field()

The reset_output_field() function is for clearing output field
settings and will be used for test code in later patch.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1400480762-22852-19-git-send-email-namhyung@kernel.org


Signed-off-by: Jiri Olsa <jolsa@kernel.org>