drm/amd/display: Move link functions from dc to dc_link

[Why]
link-specific functions should reside in dc_link.c

[How]
Move them there.

Signed-off-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>