drm/nouveau/disp/nv50-: bump max chans to 21

GP102's cursors go from chan 17..20. Increase the array size to hold
their data properly.

Fixes: e50fcff1 ("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>