media: videobuf2: use sgtable-based scatterlist wrappers

Use common wrappers operating directly on the struct sg_table objects to
fix incorrect use of scatterlists sync calls. dma_sync_sg_for_*()
functions have to be called with the number of elements originally passed
to dma_map_sg_*() function, not the one returned in sgt->nents.

Fixes: d4db5eb5 ("media: videobuf2: add begin/end cpu_access callbacks to dma-sg")
CC: stable@vger.kernel.org
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>