msm: kgsl: Enable preemption on a640v2

Graphics contexts can submit to different ringbuffers
based on their priority levels so that higher priority
workloads can finish faster without waiting for lower
priority workloads that are inflight.

Change-Id: I941a805818c1be9269679b222857ccaaee1e7227
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>