perf script: No need to lookup thread twice

We get the thread when we call perf_event__preprocess_sample(), no need
to do it before that.

Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1427906210-10519-2-git-send-email-acme@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>