msm: kgsl: Use bus_split sysfs node to disable bus DCVS

Bus_split sysfs node is used to disable or enable bus DCSV
on-the-fly. The code for checking this value was lost during
refactoring VBIF-based GPU bus BW governor. This change puts
it back.

Change-Id: I3ff3a896830655132e8749f1eafc64cd3a2cb366
Signed-off-by: Vladimir Razgulin <vrazguli@codeaurora.org>