iommu/dma: remove most stubs in iommu-dma.h

The direct calls from mapping.c all guarded by use_dma_iommu(), so don't
bother to provide stubs, but instead just expose the prototypes
unconditionally.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Leon Romanovsky <leon@kernel.org>