V4L/DVB: videobuf-dma-sg: remove external function videobuf_dma_sync()

While analyzing one of the videobuf patches, I noticed that
videobuf_dma_sync is only used internally inside videobuf-dma-sg.
So, let's remove this function, merging the code at __videobuf_dma_sync()

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>