[media] media: vb2 dma-contig: Fully cache synchronise buffers in prepare and finish

In videobuf2 dma-contig memory type the prepare and finish ops, instead of
passing the number of entries in the original scatterlist as the "nents"
parameter to dma_sync_sg_for_device() and dma_sync_sg_for_cpu(), the value
returned by dma_map_sg() was used. Albeit this has been suggested in
comments of some implementations (which have since been corrected), this
is wrong.

Fixes: 199d101e ("v4l: vb2-dma-contig: add prepare/finish to dma-contig allocator")

Cc: stable@vger.kernel.org # for v3.8 and up
Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>