vc4: Don't try to use color load/stores to do depth/stencil blits.

Fixes regressions in fbo-generatemipmap-formats on depth/stencil (which
does blits to work around baselevel/lastlevel).

src/gallium/drivers/vc4/vc4_blit.c

@@ -125,6 +125,9 @@ vc4_tile_blit(struct pipe_context *pctx, const struct pipe_blit_info *info)
 {
         struct vc4_context *vc4 = vc4_context(pctx);
 
+        if (util_format_is_depth_or_stencil(info->dst.resource->format))
+                return false;
+
         if ((info->mask & PIPE_MASK_RGBA) == 0)
                 return false;