msm: kgsl: use kgsl_mmu_device_setstate() whenever the gpu is idle

We really don't want new GPU commands or events to be generated
just to manage the iommu while we are idle.

Change-Id: I2e8740bee8c25c93bddc51a90a3370d151aaf558
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>
(cherry picked from commit 1a3279ee70f72d67c5b259573096a9953257a9b5)
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>