drm/msm/dp: store dp_display in the driver data

Store the dp_display in the platform driver data instead of the
dp_display_private.

This is required to allow other sub-modules to reuse the platform
driver data.

Changes in v3: none
Changes in v4: none
Changes in v5: none
Changes in v6: rebase on top of latest patchset of dependency

Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>