perf tools: Make 'trace' or 'trace_fields' sort key default for tracepoint events

When an evlist contains tracepoint events only, use 'trace' sort key as
default.  If --raw-trace option was given, use 'trace_fields' instead.
This will make users more convenient to see trace result.

Suggested-and-Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1450804030-29193-14-git-send-email-namhyung@kernel.org


[ Check evlist in get_default_sort_order() fixing a segfault in 'perf test hists' reported by Jiri Olsa ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>