msm: kgsl: Use BW_STEP as 50 for AB voting

AB voting is done in multiples of BW_STEP. Using a big
BW_STEP can result in AB overvoting because of roundup.
Use BW_STEP as 50 instead of 160. This will reduce AB
overvoting and will improve power.

Change-Id: Ieff338685be9650218fd7accc4ed3bcf0d2756ec
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>