drm/amd/display: move dp cts functions from dc_link_dp to link_dp_cts

Create new files link_dp_cts.c and link_dp_cts.h, and move DP cts
functions into them.

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>