drm/nouveau/fbcon/nv11-: correctly account for ring space usage

The RING_SPACE macro accounts how much space is used up so it's
important to ask it for the right amount. Incorrect accounting of this
can cause page faults down the line as writes are attempted outside of
the ring.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>