drm/amd/display: merge dc_link_dp into dc_link

[why]
Temporarly merge dc_link_dp functions into dc_link for the
purpose of removing dc_link_dp files. This is a transitional
change for later commits where we will further refactor dc_link
file.

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>