msm: kgsl: Add a barrier after writing to V2PUR register

Add a memory barrier after writing to V2PUR register of IOMMU. Without
this barrier the CPU core can stall because of multiple outstanding IOMMU
register transactions happening in parallel. This barrier synchronizes
the multiple IOMMU register transactions and prevents the CPU from
stalling.

Change-Id: I1d715e9fb8287b1f0b009b39b11b5469ab8861c8
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>