ANDROID: Export functions to be used with dma_map_ops in modules

For modules to reuse default dma_map_ops implementations they need to be
exported. Export the following functions:

dma_direct_alloc
dma_direct_free
dma_common_mmap
dma_common_get_sgtable
dma_direct_get_required_mask

Bug: 151050914
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ia77b797fcd909fce01da7431bfbde282dc70b3b3