perf evsel: Introduce perf_evsel__is_group_event() helper

The perf_evsel__is_group_event function is for checking whether given
evsel needs event group view support or not.  Please note that it's
different to the existing perf_evsel__is_group_leader() which checks
only the given evsel is a leader or a standalone (i.e. non-group) event
regardless of event group feature.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1362462812-30885-7-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>