The display buffers must be mapped for DMA through the device that
performs memory access. Expose an API to map and unmap memory through
the VSP device to be used by the DU.

As all the buffers allocated by the DU driver are coherent, we can skip
cache handling when mapping and unmapping them. This will need to be
revisited when support for non-coherent buffers will be added to the DU
driver.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
[Kieran: Remove unused header]
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Mauro Cavalho Chehab <mchehab@s-opensource.com>