iommu/ipmmu-vmsa: Unify domain alloc/free

We have two implementations for ipmmu_ops->alloc depending on
CONFIG_IOMMU_DMA, the difference being whether they accept the
IOMMU_DOMAIN_DMA type or not. However, iommu_dma_get_cookie() is
guaranteed to return an error when !CONFIG_IOMMU_DMA, so if
ipmmu_domain_alloc_dma() was actually checking and handling the return
value correctly, it would behave the same as ipmmu_domain_alloc()
anyway.

Similarly for freeing; iommu_put_dma_cookie() is robust by design.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>