iommu: arm-smmu: Correct decrement of s2cr use count

If a pair (context bank, s2cr, smr) initialized by the bootloader is handed
off to its real client, only decrement the counts of the corresponding
s2cr.

Fixes commit da68882a
("iommu: arm-smmu: Handoff SMR registers and context banks")

Change-Id: Id26f0f3b1835fd3d0707c573641b60f44f991f67
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>