perf hists browser: Only 'Zoom into thread' only when sort order has 'pid'

We can't offer a zoom into thread when a bucket (struct hist_entry) may
have samples for more than one thread, i.e. when 'pid' is not part of
the sort order, fix it.

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: Jiri Olsa <jolsa@kernel.org>,
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452960197-5323-9-git-send-email-namhyung@kernel.org


[ Carved out from a  larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>