drm/amd/display: Add Function declaration in dc_link

[WHY]
Housekeeping cleaning and adding declaration for
function to be called from DM layer

[HOW]
Adding public functions to dc_link.h

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mustapha Ghaddar <mghaddar@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>