console: fix dpy_gfx_replace_surface assert

console: fix dpy_gfx_replace_surface assert

virtio-gpu can trigger the assert added by commit "6905b93447 console:
add same surface replace pre-condition" in multihead setups (where
surface can be NULL for secondary displays).  Allow surface being NULL.

Fixes: 6905b93447a42e606dfd126b90f75f4cd3c6fe94
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170906142109.2685-1-kraxel@redhat.com
(cherry picked from commit 1540008629bbb6a9c0826582d94ecf7a559f784c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>