drm/amd/display: use proper pipe_ctx index

Use link->link_index as index to pipe_ctx[] to get proper link
information instead of using index 0 to avoid potential miss matches.

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>