drm/msm/a6xx: Send ACD state to QMP at GMU resume

The QMP mailbox expects to be notified of the ACD (Adaptive Clock
Distribution) state. Get a handle to the mailbox at probe time and
poke it at GMU resume.

Since we don't fully support ACD yet, hardcode the message to "val: 0"
(state = disabled).

Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> # sm8450
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/559287/


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