dma-mapping: reliably inform about DMA support for IOMMU

If the DMA IOMMU path is going to be used, the appropriate check should
return that DMA is supported.

Fixes: b5c58b2f ("dma-mapping: direct calls for dma-iommu")
Closes: https://lore.kernel.org/all/181e06ff-35a3-434f-b505-672f430bd1cb@notapiano


Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com> #KernelCI
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>