msm: kgsl: Correct GPU busy counter overflow detection

Correct overflow detection check to return true only if
previous value of register ADRENO_REG_RBBM_PERFCTR_RBBM_0_HI
is less than current value.

Also, update the counter variable to current counter value
incase where abnormal counter value is observed to make sure
next GPU busy sample is correct.

Change-Id: If2d504791123eedf1dca3bdc85ff0166c1e8ae13
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>