drm/virtio: move virtio_gpu_object_{attach, detach} calls.

Drop the dummy ttm backend implementation, add a real one for
TTM_PL_FLAG_TT objects.  The bin/unbind callbacks will call
virtio_gpu_object_{attach,detach}, to update the object state
on the host side, instead of invoking those calls from the
move_notify() callback.

With that in place the move and move_notify callbacks are not
needed any more, so drop them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190318113332.10900-2-kraxel@redhat.com