drm/msm/dp: define stubs for publicly accessible members

Define stubs for publicly accessible members in order to allow
compilation in the caller if DisplayPort module is not enabled
in a build. This change also helps to decouple the DisplayPort
module from the upper layers in the DRM framework.

Change-Id: Iccada0a6913e0d3f39d518252e6d30e24d92c741
Signed-off-by: Samantha Tran <samtran@codeaurora.org>