[media] videobuf2-dma-contig: allow to vmap contiguous dma buffers

This allows drivers to vmap contiguous dma buffers so they can inspect the
buffer contents with the CPU. This will be needed for the CODA driver's JPEG
handling. On CODA960, the header parsing has to be done on the CPU. The
hardware modules can only process the entropy coded segment after all
registers and tables are set up.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>