drm/msm/dp: move pdev from struct dp_display_private to struct msm_dp

The dp_drm needs accessing the DP's platform device. Move pdev to the
public structure.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/561642/


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