drm/amd/display: verify stream link before link test

[Why]
DP1.2 LL CTS test failure.

[How]
The failure is caused by not verify stream link is equal
to link, only check stream and link is not null.
Signed-off-by: Jing Zhou <Jing.Zhou@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_hwss.c

@@ -277,7 +277,8 @@ void dp_retrain_link_dp_test(struct dc_link *link,
 		if (pipes[i].stream != NULL &&
 			!pipes[i].top_pipe && !pipes[i].prev_odm_pipe &&
 			pipes[i].stream->link != NULL &&
-			pipes[i].stream_res.stream_enc != NULL) {
+			pipes[i].stream_res.stream_enc != NULL &&
+			pipes[i].stream->link == link) {
 			udelay(100);
 
 			pipes[i].stream_res.stream_enc->funcs->dp_blank(