msm: kgsl: Set default A6XX preemption level

Set the default preemption level to L1A.
Set the usesgmem bit when triggering preemption.
Also, put the preemption variables in the preemption
structure.

Change-Id: I4b74d91af9094a866058b64d4b9d217c025093fc
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>