msm: kgsl: Separate out A6xx GMU specific code

A6xx GPU has different GMUs, which requires a different
set of sequence programming. Move A6xx GMU specific code
to separate file, so that it's easy and scalable to fit
new kind of GMU in existing driver.

Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>
Change-Id: I7773b1626a8fa6036dda06753466d2c822960096