nvc0: fix queries which use multiple MP counters on Fermi

nvc0: fix queries which use multiple MP counters on Fermi

Queries which use more than one MP counters was misconfigured and
computing the final result was also wrong because sources need to
be configured on different hardware counters instead.

According to the blob, computing the result is now as follows:

FOR  i..n
val += ctr[i] * pow(2, i)

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