perf top: Update event name when falling back to cpu-clock

The 'perf top' command falls back to cpu-clock if the H/W cycles event
is not supported, but the event name is not updated leading to a
misleading header:

PerfTop: 8 irqs/sec  kernel:75.0%  exact:  0.0% [1000Hz cycles],  ...

Update the event name when the event type is changed so that the
header displays correctly:

PerfTop: 794 irqs/sec  kernel:100.0%  exact:  0.0% [1000Hz cpu-clock], ...

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1336495789-58420-1-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>