msm: kgsl: Add barrier to read preemption state correctly

Preemption state gives us the current state of preemption.
This state is updated in different points in the driver.
Without a barrier we are reading stale values, add a barrier
to read the most recent updated preemption state.

Change-Id: I1e52b2c3c6cd8aec5252c6e750a7012c68ab26dd
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>