wayland: Realize dmabuf buffers before trying to attach them

Commit 22723ca37 moved buffer realization to
meta_wayland_surface_commit() so that it wouldn't be part of
meta_wayland_buffer_attach().

However, creation of dmabuf buffers would call into
meta_wayland_buffer_attach() directly without realizing the buffer
first. attach() would then fail and mutter would effectively shut down
any clients using the zwp_linux_dmabuf protocol (note that if such
client was Xwayland, mutter itself would shut down as well).

Add the missing bit in order to make zwp_linux_dmabuf protocol work
again.

(cherry picked from commit 54709c16b5)

src/wayland/meta-wayland-dma-buf.c

@@ -365,6 +365,7 @@ buffer_params_create_common (struct wl_client   *client,
                                   dma_buf, NULL);
   buffer = meta_wayland_buffer_from_resource (buffer_resource);
 
+  meta_wayland_buffer_realize (buffer);
   if (!meta_wayland_buffer_attach (buffer, &error))
     {
       if (buffer_id == 0)