V4L/DVB (11731): buf-dma-contig: remove sync operation

Remove the videobuf-dma-contig sync operation. Sync is only needed
for noncoherent buffers, and since videobuf-dma-contig is built on
coherent memory allocators the memory is by definition always in sync.

Reported-by: Matthieu CASTET <matthieu.castet@parrot.com>
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Reviewed-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>