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: 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>

drivers/gpu/drm/nouveau/nvkm/engine/disp/nv50.h

@@ -22,7 +22,7 @@ struct nv50_disp {
 		u8 type[3];
 	} pior;
 
-	struct nv50_disp_chan *chan[17];
+	struct nv50_disp_chan *chan[21];
 };
 
 void nv50_disp_super_1(struct nv50_disp *);