msm: kgsl: Enable A6XX preemption through sysfs

This patch enables the a6xx preemption buffers setup by default.
However, the preemption execution is only enabled by:

echo 1 > /sys/class/kgsl/kgsl-3d0/preemption

Change-Id: I32c513fd5213f949ab6bc99fafbe71dc6a6f332b
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>