iommu/dma: Don't depend on CONFIG_DMA_DIRECT_REMAP

For entirely dma coherent architectures there is no requirement to ever
remap dma coherent allocation.  Move all the remap and pool code under
IS_ENABLED() checks and drop the Kconfig dependency.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>