v4l2: videobuf2-dc: fix support for mappings without struct page in userptr mode

v4l2: videobuf2-dc: fix support for mappings without struct page in userptr mode

Earlier version of dma-contig allocator in user ptr mode assumed that in
all cases DMA address equals physical address. This was just a special case.
Commit e15dab752d4c588544ccabdbe020a7cc092e23c8 introduced correct support
for converting userpage to dma address, but unfortunately it broke the
support for simple dma address = physical address for the case, when given
physical frame has no struct page associated with it (this happens if one
use for example dma_declare_coherent api or other reserved memory approach).
This commit restores support for such cases.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>