drm/amd/display: move dpcd logic from dc_link_dpcd to link_dpcd

[why]
Moving dpcd logic from dc_link_dpcd to link_dpcd as part of link file
restructure

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