Add the Display Port controller subnode to the MDSS node.

Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> #SM8350-HDK
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230206-topic-sm8450-upstream-dp-controller-v6-3-d78313cbc41d@linaro.org