msm: kgsl: Update preemption keepalive on a660

A660 has a new dedicated register to assert preemption
keepalive.

Change-Id: I776fc18405b6c9b827430373adcfe8582257343a
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>
Signed-off-by: Nitheesh Muthuraj <nmuthuraj@codeaurora.org>