drm/msm/dp: support setting the DP subconnector type

Read the downstream port info and set the subconnector type accordingly.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/555531/
Link: https://lore.kernel.org/r/20230903222432.2894093-1-dmitry.baryshkov@linaro.org