mesa: don't double incr/decr ActiveCounters

mesa: don't double incr/decr ActiveCounters

Frameretrace ends up w/ excess calls to SelectPerfMonitorCountersAMD()
which ends up re-enabling already enabled counters.  Which causes
ActiveCounters[group] to be double incremented for the same counter.
This causes BeginPerfMonitorAMD() to fail.

The AMD_performance_monitor spec doesn't say that an error should be
generated in this case.  So I think the safe thing to do is just safe-
guard against excess increments/decrements.

Signed-off-by: Rob Clark <robdclark@gmail.com>