drm/nouveau: oops, increase channel dispc_vma to 4

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

drivers/gpu/drm/nouveau/nouveau_drv.h

@@ -294,7 +294,7 @@ struct nouveau_channel {
 
 	uint32_t sw_subchannel[8];
 
-	struct nouveau_vma dispc_vma[2];
+	struct nouveau_vma dispc_vma[4];
 	struct {
 		struct nouveau_gpuobj *vblsem;
 		uint32_t vblsem_head;