drm/amd/display: Handle virtual signal type in disable_link()

[Why]
The new implementation changed the behavior to allow process setMode
to DAL when DAL returns empty mode query for unplugged display.
This will trigger additional disable_link().
When unplug HDMI from MST dock, driver will update stream->signal to
"Virtual". disable_link() will call disable_output() if the signal type
is not DP and induce other displays on MST dock show black screen.

[How]
Don't need to process disable_output() if the signal type is virtual.
Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -2168,8 +2168,10 @@ static void disable_link(struct dc_link *link, enum signal_type signal)
 			dp_set_fec_ready(link, false);
 		}
 #endif
-	} else
-		link->link_enc->funcs->disable_output(link->link_enc, signal);
+	} else {
+		if (signal != SIGNAL_TYPE_VIRTUAL)
+			link->link_enc->funcs->disable_output(link->link_enc, signal);
+	}
 
 	if (signal == SIGNAL_TYPE_DISPLAY_PORT_MST) {
 		/* MST disable link only when no stream use the link */