msm: kgsl: Fix preemption fault handling for A6xx GPU

In preemption fault handler, check only the STOP bit of
ADRENO_REG_CP_PREEMPT register to see if the preemption is complete.

Change-Id: Id1f2d0947412e996d6028558124c3e528527cec6
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Sebanti Das <sebadas@codeaurora.org>