intel/nir: also allow unknown format for getting the size of a storage image

Fixes: fa251cf111 ("intel/nir: allow unknown format in lowering of storage images")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13847>

src/intel/compiler/brw_nir_lower_storage_image.c

@@ -646,6 +646,9 @@ lower_image_size_instr(nir_builder *b,
    if (var->data.access & ACCESS_NON_READABLE)
       return false;
 
+   if (var->data.image.format == PIPE_FORMAT_NONE)
+      return false;
+
    /* If we have a matching typed format, then we have an actual image surface
     * so we fall back and let the back-end emit a TXS for this.
     */