msm: kgsl: Set correct context restore address during preemption

On A6XX, when triggering preemption, we need to tell CP from where
to restore the incoming context's saved data. Currently, the logic
to figure out the incoming context is incorrect. Fix it by wrapping
the IBs with commands to write the context's restore address to
scratch memory location. The address is then read from the scratch
memory location at the time of triggering preemption.

Change-Id: Ie83bd1adc0f24faa122748b972fad28e0d05b0e6
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>