perf evlist: Introduce evsel list accessors

To replace the longer list_entry constructs for things that are widely
used:

	perf_evlist__{first,last}(evlist)
	perf_evsel__next(evsel)

Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@gmail.com>
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-ng7azq26wg1jd801qqpcozwp@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/top.c b/tools/perf/util/top.c
index 7eeebce..884dde9 100644
--- a/tools/perf/util/top.c
+++ b/tools/perf/util/top.c
@@ -58,8 +58,7 @@
 	}
 
 	if (top->evlist->nr_entries == 1) {
-		struct perf_evsel *first;
-		first = list_entry(top->evlist->entries.next, struct perf_evsel, node);
+		struct perf_evsel *first = perf_evlist__first(top->evlist);
 		ret += SNPRINTF(bf + ret, size - ret, "%" PRIu64 "%s ",
 				(uint64_t)first->attr.sample_period,
 				top->freq ? "Hz" : "");