drm/amd/display: Expose generic SDP message access interface

[Why]
We need to add DP SDP message test debugfs to make sdp message test
more convenient and efficient.

[How]
Add a DM accessible SDP interface for custom data.

Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>