nouveau: add interface to call an object's methods

This will expose functionality supported by newer kernel interfaces,
giving access to things such as ZBC controls, perfmon, etc.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>