media: rockchip: rga: extract helper to fill descriptors

The IOMMU of the RGA is programmed with a list of DMA descriptors that
contain an 32 bit address per 4k page in the video buffers. The address
in the descriptor points to the start address of the page.

Introduce 'struct rga_dma_desc' to make the handling of the DMA
descriptors explicit instead of hiding them behind standard types.

While at it, use provided helpers for iterating the sg_table instead of
manually calculating the DMA addresses.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>