msm: kgsl: Make all GMU code target specific

Move all the GMU code to a6xx target specific code. This is the first in
several steps to make the GMU code more modular and flexibile for
various usecases.

Change-Id: Ic0dedbad0e70771c301705a0eb4ed422efb4c80a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>