drm/amd/display: Fix system hang at boot

[Why]
During DQE's promotion test, system hang issue is found on linux
system;

[How]
1. Add NULL pointor check for the link in the sequence trace
   function;
2. Get the right link for the stream encoder before blank DP
   stream;

Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>