iommu/arm-smmu-v3: Separate s/w and h/w views of prod and cons indexes

In preparation for rewriting the command queue insertion code to use a
new algorithm, separate the software and hardware views of the prod and
cons indexes so that manipulating the software state doesn't
automatically update the hardware state at the same time.

No functional change.

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