msm: kgsl: Restrict preemption enablement through sysfs

Do not enable preemption through sysfs if it was
not enabled in the device specific features. Let's
say preemption is not enabled during probe. Then
we will not have any preemption buffers allocated
and set up. Also, we would only probe one ringbuffer.
And now, through sysfs if we enable preemption, it
will lead to mayhem. To avoid that, modify preemption
only if it was enabled during probe time.

Change-Id: Ie9e799a4323897aac7095665b24a6a6d437f9f28
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>