perfmon: destroy the engine context

This fixes a crash when we reload Nouveau.
abi16: expose NV_PERFCTR_CLASS using a ioctl

This allows the userspace to allocate perfctr objects in order to
monitor performance counters through the perfmon engine.
sw/nv50: add an interface to control perfmon

This will be used to monitor global performance counters (PCOUNTER) from
the userspace with a set of software methods.