perf hists: Separate sort fields parsing into setup_sort_list function

perf hists: Separate sort fields parsing into setup_sort_list function

Separating sort fields parsing into setup_sort_list function, so it's
separated from sort_order string setup and could be reused later in
following patches.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1453109064-1026-13-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>