arch-arm: when programming an invalid PMU ID detach the counter (#1510)

I'm not entirely sure what the mandated behaviour is according the the
ARM ARM, but I was very confused by the counters continuing to increment
with the old event even when programmed to an event ID that is not
currently supported by GEM5. Disconnecting the counter if the event is
not supported is less surprising behaviour IMO.

Change-Id: I927d9339c138dafa1484db1515c2aa09b0a9a0a9

src/arch/arm/pmu.cc

@@ -601,6 +601,7 @@ PMU::updateCounter(CounterState &ctr)
         if (sourceEvent == eventMap.end()) {
             warn("Can't enable PMU counter of type '0x%x': "
                  "No such event type.\n", ctr.eventId);
+            ctr.detach();
         } else {
             ctr.attach(sourceEvent->second);
         }