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
00e4db51259a5f936fec1424b884f029479d3981 "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>