iommu/arm-smmu-v3: Encapsulate ctx_desc_cfg init in alloc_cd_tables

iommu/arm-smmu-v3: Encapsulate ctx_desc_cfg init in alloc_cd_tables

This is slighlty cleaner: arm_smmu_ctx_desc_cfg is initialized in a
single function instead of having pieces set ahead-of time by its caller.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Michael Shavit <mshavit@google.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/20230915211705.v8.3.I875254464d044a8ce8b3a2ad6beb655a4a006456@changeid
Signed-off-by: Will Deacon <will@kernel.org>