iommu/arm-smmu-v3: Rework enabling/disabling of ATS for PCI masters

To prevent any potential issues arising from speculative Address
Translation Requests from an ATS-enabled PCIe endpoint, rework our ATS
enabling/disabling logic so that we enable ATS at the SMMU before we
enable it at the endpoint, and disable things in the opposite order.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>