msm: kgsl: Correctly mask out the OOB bits

The result of GENMASK needs to be inverted so that the
handler doesn't report OOB bits as unknown bits.

Change-Id: I3e39334a6dd79343109591b043c9aa30a8bdff9d
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>