drm/msm/a6xx: Improve GMU force shutdown sequence

The GMU force shutdown sequence involves some additional register cleanup
which was not implemented previously. Do so.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543340/


Signed-off-by: Rob Clark <robdclark@chromium.org>