drm/amd/display: Add stereo mux and dig programming calls for dcn21

[Why]
The earlier patch "Hook up calls to do stereo mux and dig programming..."
doesn't include update for dcn21.

[How]
Align dcn21 gpio settings with updated stereo control interface.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Acked-by: Aaron Liu <aaron.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>