drm/msm/a4xx: Implement .gpu_busy

Add support for gpu_busy on a4xx, which is required for devfreq
support.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/523791/
Link: https://lore.kernel.org/r/20230223-topic-opp-v3-6-5f22163cd1df@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>