imx-drm: ipu-dp: Introduce IPUV3_NUM_FLOWS

IPUv3 has a total of 3 flows (one synchronous flow and 2 asynchronous flows).

Let's add a definition for such number in order to let the code easier to
understand.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>