iommu/amd: Remove dev == NULL checks

This is no longer possible, amd_iommu_domain_alloc_paging_flags() is never
called with dev = NULL from the core code. Similarly
get_amd_iommu_from_dev() can never be NULL either.

Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v2-9776c53c2966+1c7-amd_paging_flags_jgg@nvidia.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>