drm/nouveau: inform userspace of new kernel subchannel requirements

All available subchannels are now available for userspace to do with as it
pleases on NVC0+.

On all earlier chipsets, the kernel still uses a software object on subc 0
to implement the page flip completion method.  I hope to find some decent
way of addressing this too, but it's a tad tricker prior to fermi.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>