msm: kgsl: Add target specific clock and bus scaling

This allows us to define target specific clock and bus scaling
functions. Also, get rid of the dcvs_set function pointer
from gmu core ops.

Change-Id: I137ea0071958bfdd73a266d734e278703f0dc722
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>