dma-mapping: always build the direct mapping code

All architectures except for sparc64 use the dma-direct code in some
form, and even for sparc64 we had the discussion of a direct mapping
mode a while ago.  In preparation for directly calling the direct
mapping code don't bother having it optionally but always build the
code in.  This is a minor hardship for some powerpc and arm configs
that don't pull it in yet (although they should in a relase ot two),
and sparc64 which currently doesn't need it at all, but it will
reduce the ifdef mess we'd otherwise need significantly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>