perf evsel: No need to always ask for PERF_FORMAT_ID in read_format

Instead make perf_evlist__confir_attrs use perf_evsel__set_sample_id()
when having more than one event, that way only if we have multiple
events we'll ask to have the event ids returned when we read its file
descriptors.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-xuho5hrrxy2ky0cjpr80hyfp@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>