metag: perf: fix frequency sampling (dynamic period)

Frequency sampling mode dynamically adjusts the sample period so as to
hit a particular frequency of samples. The sample period starts at just
1 and then gets increased if the interrupt rate is too high. This
changed sample period needs handling in metag_pmu_event_set_period to
update period_left (as the ARM equivalent does). The calculated delta
also needs subtracting from period_left in metag_pmu_event_update in
order to hit the conditional blocks in metag_pmu_event_set_period which
update last_period (which is used in the dynamic sampling period
calculation).

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>