drm/amd/display: Removing Unused DPP Functions

[Why & How]
The functions in this commit are defined for dpp401 but never used.
Removing them as they are not necessary.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Ryan Seto <ryanseto@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>