This commit negotiates a common protocol version with the clients,
allowing for backwards compatibility on both sides.

v2: - Negotiate the version in a new command, so the client can probe
      for its existence and stay compatible with older servers. (Dave
      Airlie)
Tested-by: Jakob Bornecrantz <jakob@collabora.com>
Reviewed-by: Jakob Bornecrantz <jakob@collabora.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>