video: fbdev: fsl: Fix kernel crash when diu_ops is not implemented

If diu_ops is not implemented on platform, kernel will access a NULL
pointer. We need to check this pointer in DIU initialization.

Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
Acked-by: Timur Tabi <timur@tabi.org>
Cc: stable@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>