drm/amd/display: Generic SDP message access in amdgpu

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

[How]
Add sdp_message debugfs entry in amdgpu.

Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>