media: imx: imx7-media-csi: Split imx7_csi_dma_stop()

The imx7_csi_dma_stop() function performs two operations, it waits for
DMA to be complete, and then cleans up and frees buffers. Split the
second part out to a new imx7_csi_dma_cleanup() function to match
imx7_csi_dma_setup(), in preparation for a stream start refactoring.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>