perf tools: Move all users of event_name to perf_evsel__name

So that we don't use global variables that could make us misreport event
names when having a multi window top, for instance.

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-mccancovi1u0wdkg8ncth509@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>