drm/nouveau/gr/gf117-: make ppc_nr[gpc] accurate

We're going to be pulling in a chunk of code from NVGPU to fixup our
SMID mappings on Volta and above, which depends on ppc_nr[gpc]
reflecting the actual number of PPCs present, not the maximum number.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>