msm: kgsl: Add a path for dynamic AB bus votes

Modify the power control code and vbif_bw governor
to allow dynamic AB votes to be set with each GPU
IB vote.  As a placeholder set a static AB vote of
one fourth the IB vote.

Change-Id: I3edb430f9a4545f008ec81b75ddff3c717e91e39
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>