Revert "msm: kgsl: Enable A6XX preemption through sysfs"

This reverts commit 38e57d7a.

1. This removes the bifurcation of preemption setup and execution
paths. Preemption is stable and can both setup and execution
can be enabled.
2. Currently preemption is enabled for all devices. Remove that
and selectively enable preemption for a630 in device features in
adreno-gpulist.h

Change-Id: I5e72b1a62bcb6ecffb655d41d1597655a3f11a0c
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>