msm: kgsl: Implement mutex between SMMU operations and GPU power collapse

Implement mutex between SMMU operations and GPU power collapse
to prevent a glitch between GPU and SMMU on 8996v2. This mutex
prevents a condition where GPU vreg gets turned on/off at the
same time as SMMU is turning on/off GPU CX vreg.

Change-Id: I3011f9850ac0eb393d4ba6765c5803ea380013fd
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>