drm/amd/display: Remove duplicate HWSS interfaces

[Why]
Some interface functions are defined in both the public and private HWSS
interfaces, which can lead to confusion and runtime issues, therefore
the duplicates should be eliminated.

[How]
- power_down should only be private, because it's only used within HWSS.
- update_plane_addr should only be public, as it's used outside HWSS.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>