perf callchain: Move initial entry call into get_entries function

perf callchain: Move initial entry call into get_entries function

Moving initial entry call into get_entries function so all entries
processing is on one place. It will be useful for next change that adds
ordering logic.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Milian Wolff <milian.wolff@kdab.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1447772739-18471-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>