drm/msm/dpu: simplify ctl_setup_blendstage calculation

Extract the common expression in the dpu_hw_ctl_setup_blendstage()
function.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550
Patchwork: https://patchwork.freedesktop.org/patch/518483/
Link: https://lore.kernel.org/r/20230116063316.728496-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>