nvc0: allow only one active query for the MP counters group

nvc0: allow only one active query for the MP counters group

Because we can't expose the number of hardware counters needed for each
different query, we don't want to allow more than one active query
simultaneously to avoid failure when the maximum number of counters
is reached. Note that these groups of GPU counters are currently only
used by AMD_performance_monitor.

Like for Kepler, this limits the maximum number of active queries
to 1 on Fermi.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>