drm/amd/display: Fix ASSR regression on embedded panels

[Why]
Regression found in some embedded panels traces back to the earliest
upstreamed ASSR patch. The changed code flow are causing problems
with some panels.

[How]
- Change ASSR enabling code while preserving original code flow
  as much as possible
- Simplify the code on guarding with internal display flag

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=213779
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1620
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

Author: Stylon Wang

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

@@ -1820,8 +1820,7 @@ bool perform_link_training_with_retries(
 					 */
 					panel_mode = DP_PANEL_MODE_DEFAULT;
 				}
-			} else
-				panel_mode = DP_PANEL_MODE_DEFAULT;
+			}
 		}
 #endif
 
@@ -4650,7 +4649,10 @@ enum dp_panel_mode dp_get_panel_mode(struct dc_link *link)
 		}
 	}
 
-	if (link->dpcd_caps.panel_mode_edp) {
+	if (link->dpcd_caps.panel_mode_edp &&
+		(link->connector_signal == SIGNAL_TYPE_EDP ||
+		 (link->connector_signal == SIGNAL_TYPE_DISPLAY_PORT &&
+		  link->is_internal_display))) {
 		return DP_PANEL_MODE_EDP;
 	}