GP102's cursors go from chan 17..20. Increase the array size to hold
their data properly.
Fixes: e50fcff15f ("drm/nouveau/disp/gp102: fix cursor/overlay immediate channel indices")
Cc: stable@vger.kernel.org # v4.10+
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>