[media] 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 e15dab75 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>
[s.nawrocki@samsung.com: replaced #elsif with #elif]
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>