drm/amd/display: Rename header file link.h to link_service.h

[WHY]
Header file name "link.h" collides with system header when dc is
compiled as a user-mode library

[WHAT]
Rename link.h to link_service.h to avoid name collision

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>