msm: kgsl: Fix the preemption sysfs node

The preemption sysfs node does not change the assigned ringbuffer
for existing draw contexts, so turning preemption from
on to off does not work. Add a loop that goes through
each context and updates the draw contexts to fix this.

Change-Id: I9c5b3bcdc4cfd70f5c1da6f6458b9e1290c0444f
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>