msm: iommu: Lock the genpool allocation

Previously, IOMMU allocation and mapping was handled through Ion.
Ion provided very coarse locking that essentially serialized access
to iova allocation. Now that IOMMU allocation is out of Ion,
the genpool allocation for IOVAs needs to be serialized again.
Take the pool lock for each IOVA pool to prevent against races.

Change-Id: I6d38995549cabd3d082697e440e3aafc1bdcfaa5
CRs-Fixed: 481328
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>