nvc0: use NOUVEAU_GETPARAM_GRAPH_UNITS to get MP count
nvc0: implement MP performance counters

There's more, but this only adds (most) of the counters that are
handled directly by the shader processors.
The other counter domains are not handled on the multiprocessor and
there are no FIFO object methods for configuring them.
Instead, they have to be programmed by the kernel via PCOUNTER, and
the interface for this isn't in place yet.