perf tools: Reset output/sort order to default

When reset_output_field() is called, also reset field/sort order to
NULL so that it can have the default values.  It's needed for testing.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
CC: Arun Sharma <asharma@fb.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1401335910-16832-26-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>