i965: Fix extra return value after winsys rb update refactor.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75172
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -738,7 +738,7 @@ intel_update_winsys_renderbuffer_miptree(struct brw_context *intel,
                                                           num_samples);
    if (!multisample_mt) {
       intel_miptree_release(&singlesample_mt);
-      return NULL;
+      return;
    }
 
    irb->need_downsample = false;