drm/amd/display: Drop unused privacy_mask setters and getters

[Why and How]
dwbc_funcs.set/get_privacy_mask isn't being used anymore, drop it

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Oliver Logush <oliver.logush@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>