drm/virtio: remove prime pin/unpin callbacks.

virtio-gpu objects never move around, so effectively they are
pinned all the time.  Therefore we don't need the (optional)
pin/unpin callbacks.  Remove them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190227144441.6755-3-kraxel@redhat.com