drm/amd/display: Hook up calls to do stereo mux and dig programming to stereo control interface

[Why]
Implementation of stereo mux register is complete, but unused. Need to
call functions to write relevant configs.

[How]
Add function to write stereo config for enable/disable case and call in
stereo control interface.

Signed-off-by: Murton Liu <murton.liu@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>