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

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 <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>,
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ Carved out from a  larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>