drm/msm/dp: add a separate header for register definitions

Add a separate header file for DisplayPort register definitions
so that other modules can access these definitions.

CRs-Fixed: 2057884
Change-Id: Ie68489e8056036f4656413f691540533628f2f37
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>