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>