msm: kgsl: Check if the iommu_device per ctxt is attached

Check if the iommu_device for each context is attached before
trying to read/write to any context registers. This is to prevent
us from writing to CB2 registers which have been locked down by
TZ and would generate XPU violations.

Change-Id: I202d9565625537025286386ba34e20da7e77277e
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>