i965: Don't forget to free the old singlesample_mt.

Fixes a memory leak with MSAA winsys buffers since my move of
singlesample_mt to the rb in 4e0924c5de

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

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

@@ -682,6 +682,7 @@ intel_update_winsys_renderbuffer_miptree(struct brw_context *intel,
    int num_samples = rb->NumSamples;
 
    intel_miptree_release(&irb->mt);
+   intel_miptree_release(&irb->singlesample_mt);
 
    /* Only the front and back buffers, which are color buffers, are allocated
     * through the image loader.