msm: iommu: Re-use ASIDs within an IOMMU

Reuse ASID values within an SMMU if multiple contexts
within that SMMU are using the same page table. This
should improve TLB performance by eliminating duplicate
TLB entries for the same page table but with different ASID
tags.

Change-Id: I61e3c4c3edcbc42abab0e64dcecab3d8744dc3d6
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>