msm: kgsl: Abstract out GMU from the GMU core

A6xx GPUs have different variants of GMU. Abstract
out current GMU implementation under GMU core,
so that it’s easy to add new code for different
GMU variants.

Change-Id: I0dae15bc6905b869c34bbbdebc0b5e9e2f7ff25b
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>