drm/nvc0: reserve only subc 0 for kernel use

Current 3D driver expects this behaviour.  While this could be changed,
there's no compelling reason to reserve more than one subchannel for the
DRM.  If we ever need to use an object other then M2MF, we can just
re-bind subchannel 0 as required.

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