nv30: add missing fallthrough comment

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5705>
diff --git a/src/gallium/drivers/nouveau/nv30/nv30_texture.c b/src/gallium/drivers/nouveau/nv30/nv30_texture.c
index dfea6de..5d7505d 100644
--- a/src/gallium/drivers/nouveau/nv30/nv30_texture.c
+++ b/src/gallium/drivers/nouveau/nv30/nv30_texture.c
@@ -240,6 +240,7 @@
       break;
    case PIPE_TEXTURE_CUBE:
       so->fmt |= NV30_3D_TEX_FORMAT_CUBIC;
+      /* fallthrough */
    case PIPE_TEXTURE_2D:
    case PIPE_TEXTURE_RECT:
       so->fmt |= NV30_3D_TEX_FORMAT_DIMS_2D;