radeonsi: add num_layers variable into si_do_fast_color_clear

in preparation for the next commit

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9795>

src/gallium/drivers/radeonsi/si_clear.c

@@ -461,9 +461,11 @@ static void si_do_fast_color_clear(struct si_context *sctx, unsigned *buffers,
       if (sctx->chip_class >= GFX9 && tex->buffer.b.b.last_level > 0)
          continue;
 
+      unsigned num_layers = util_num_layers(&tex->buffer.b.b, level);
+
       /* the clear is allowed if all layers are bound */
       if (fb->cbufs[i]->u.tex.first_layer != 0 ||
-          fb->cbufs[i]->u.tex.last_layer != util_max_layer(&tex->buffer.b.b, 0)) {
+          fb->cbufs[i]->u.tex.last_layer != num_layers - 1) {
          continue;
       }