ARM mx3fb: check for DMA engine type

We have two dma engines in MX3 systems: The IPU and the SDMA
engine. We have to check if we got a channel from the correct
engine before proceeding with a channel.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>