drm/amd/display: Don't skip link training for empty dongle

[Why]
Skipping initial link training will result in no verified link cap for
mode enumeration. Some versions of the BIOS seem to have PHY programming
sequence issue as well if initial link training is skipped, resulting in
a softlock in BIOS command table.

[How]
Identify the empty dongle hotplug case, and still do initial link
training.
Signed-off-by: NEric Yang <Eric.Yang2@amd.com>
Reviewed-by: NTony Cheng <Tony.Cheng@amd.com>
Acked-by: NLeo Li <sunpeng.li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -706,12 +706,26 @@ bool dc_link_detect(struct dc_link *link, enum dc_detect_reason reason)
 				if (memcmp(&link->dpcd_caps, &prev_dpcd_caps, sizeof(struct dpcd_caps)))
 					same_dpcd = false;
 			}
-			/* Active dongle downstream unplug */
+			/* Active dongle plug in without display or downstream unplug*/
 			if (link->type == dc_connection_active_dongle
 					&& link->dpcd_caps.sink_count.
 					bits.SINK_COUNT == 0) {
-				if (prev_sink != NULL)
+				if (prev_sink != NULL) {
+					/* Downstream unplug */
 					dc_sink_release(prev_sink);
+				} else {
+					/* Empty dongle plug in */
+					for (i = 0; i < LINK_TRAINING_MAX_VERIFY_RETRY; i++) {
+						int fail_count = 0;
+
+						dp_verify_link_cap(link,
+								  &link->reported_link_cap,
+								  &fail_count);
+
+						if (fail_count == 0)
+							break;
+					}
+				}
 				return true;
 			}