msm: kgsl: Fix gmu fence error during slumber

There can exist a race where we clear the A6XX_RBBM_INT_0_MASK
register while the gpu is going into IFPC. Fix this by clearing this
register before clearing the OOB.

Change-Id: I08e9dbccbfdbee93ba7ef35fbbc673938ee033da
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>