By default dma_map_sg syncs the mapped buffer to the device. But
buf_prepare expects a buffer syncs for the cpu and the buffer
will be synced to the device in the prepare memop.

The reverse is true for dma_unmap_sg, buf_finish and the finish
memop.

To prevent unnecessary syncs we ask dma_(un)map_sg to skip the
sync.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>