msm: kgsl: Idle GPU after IOMMU V1 TLB flush

After TLB flush on IOMMU V1, stall prefetch parser until
GPU is idle, this guarantees GPU is completely idle
after executing TLB flush instructions. This prevents
prefetching GPU instructions which could trigger hardware
page table walk while TLB flush instructions are
still being executed by GPU.

CRs-Fixed: 541950
Change-Id: Iaf5e577beb640bc42b9a1e29c96d06b125629860
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>