drm/nouveau/kms/nv50-: fix bug preventing non-vsync'd page flips

Accidentally introduced during Volta bring-up.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv50/wndw.c

@@ -127,7 +127,7 @@ void
 nv50_wndw_flush_set(struct nv50_wndw *wndw, u32 *interlock,
 		    struct nv50_wndw_atom *asyw)
 {
-	if (interlock) {
+	if (interlock[NV50_DISP_INTERLOCK_CORE]) {
 		asyw->image.mode = 0;
 		asyw->image.interval = 1;
 	}