msm: kgsl: bus modifier computation

Use the vbif performance counter which counts bus busy cycles
to determine the requested bus bandwidth vote at a given
GPU frequency.

Change-Id: I18915ef8a2be75a7ef5795a6030a1f2ddd09a967
CRs-fixed: 574420
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>