iommu/arm-smmu-v3: Operate directly on low-level queue where possible

In preparation for rewriting the command queue insertion code to use a
new algorithm, rework many of our queue macro accessors and manipulation
functions so that they operate on the arm_smmu_ll_queue structure where
possible. This will allow us to call these helpers on local variables
without having to construct a full-blown arm_smmu_queue on the stack.

No functional change.

Tested-by: Ganapatrao Kulkarni <gkulkarni@marvell.com>
Signed-off-by: Will Deacon <will@kernel.org>