drm/amd/display: Remove unused DPCD declarations

[Why & How]
These DPCD addresses are either declared in other header files
where it makes more sense or simply not used by any DC code.
Remove them to reduce redundancies and potential confusion.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>