msm: kgsl: Set preemption OOB when reading the write pointer

OOB is needed because the GPU maybe in IFPC. And reading a fenced
register in FENCE drop mode causes a FENCE error interrupt and
the read returns with the signature value. For the read to go
through, the GPU must be ON. For this, add a new OOB bit to be
used exclusively for the preemption path.

Change-Id: I458559b74bf6f5438e0c77efbbdf9ae11615ee13
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>