drm/msm/dpu: create a dpu_hw_clk_force_ctrl() helper

Add an helper to setup the force clock control as it will
be used in multiple HW files.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/562323/


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