dma: ipu: remove the use of ipu_platform_data

The struct ipu_platform_data is used by platform code to pass
MXC_IPU_IRQ_START to ipu-core driver.  We can save it by having
ipu-core driver call irq_alloc_descs to get the irq_base.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>