perf evlist: Return first evsel for non-sample event on old kernel

On old kernels that don't support sample_id_all feature,
perf_evlist__id2evsel() returns NULL for non-sampling events.

This breaks perf top when multiple events are given on command line. Fix
it by using first evsel in the evlist. This will also prevent getting
the same (potential) problem in such new tool/ old kernel combo.

Suggested-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1329702447-25045-1-git-send-email-namhyung.kim@lge.com
Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>