videobuf2: replace a layering violation with dma_map_resource

vb2_dc_get_userptr pokes into arm direct mapping details to get the
resemblance of a dma address for a a physical address that does is
not backed by a page struct.  Not only is this not portable to other
architectures with dma direct mapping offsets, but also not to uses
of IOMMUs of any kind.  Switch to the proper dma_map_resource /
dma_unmap_resource interface instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>