drm/amd/display: remove unused functions

[why]
It has been decided that opm state query support will be dropped.
Therefore link encryption enabled and save current encryption states
won't be used anymore and there are no foreseeable usages in the future.
We will remove these two interfaces for clean up.

Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>