drm/msm/a6xx: Introduce a6xx_llc_read

Add a helper that does exactly what it says on the can, it'll be
required for A7xx.

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/551828/


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