msm: iommu: serialize GPU-GPU access to context registers

1. We need this to run with the new QdevNet adreno blobs.

2. HACK: prevent APPS side locking;  it's aggrevating the modem wdog.
Not sure why this happens, perhaps cpuidle, or preemption is
somehow responsible for the lock being held longer than anticipated.

3. Result is: serialized GPU-GPU access to the context registers.
Apparently this is all that is needed to be able to use the QdevNet
adreno blobs.

Change-Id: I7468196dbaefbc03977ced41ee1cef9f0b170e6c
Linux: 3.4.1