perf stat: Update POWER9 metrics to utilize other metrics

perf stat: Update POWER9 metrics to utilize other metrics

These changes take advantage of the new capability added in merge commit
79382f10352fa8492224e086b5c5d6d008e00647 "Allow using computed metrics
in calculating other metrics".

The net is a simplification of the expressions for a handful of metrics,
but no functional change.

Signed-off-by: Paul Clarke <pc@us.ibm.com>
Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: http://lore.kernel.org/lkml/20200813222155.268183-1-pc@us.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>