msm: kgsl: Set the abnormal power perf counter value to zero

During preemption microcode does save restore for all perf
counters. If we read the power counters at preemption boundary
we might get abnormal value from the perf counter. This will
result in showing incorrect GPU busy percentage. Fix this by
setting the abnormal power perf counter value with zero.

Change-Id: I96ba367ceeeb92d6adb507d0d917113297b4b58d
Signed-off-by: Abhilash Kumar <krabhi@codeaurora.org>