drm/msm/a6xx: Introduce GMU wrapper support

drm/msm/a6xx: Introduce GMU wrapper support

Some (particularly SMD_RPM, a.k.a non-RPMh) SoCs implement A6XX GPUs
but don't implement the associated GMUs. This is due to the fact that
the GMU directly pokes at RPMh. Sadly, this means we have to take care
of enabling & scaling power rails, clocks and bandwidth ourselves.

Reuse existing Adreno-common code and modify the deeply-GMU-infused
A6XX code to facilitate these GPUs. This involves if-ing out lots
of GMU callbacks and introducing a new type of GMU - GMU wrapper (it's
the actual name that Qualcomm uses in their downstream kernels).

This is essentially a register region which is convenient to model
as a device. We'll use it for managing the GDSCs. The register
layout matches the actual GMU_CX/GX regions on the "real GMU" devices
and lets us reuse quite a bit of gmu_read/write/rmw calls.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542766/
Signed-off-by: Rob Clark <robdclark@chromium.org>