drm/bridge: panel: Fix device link for DRM_BRIDGE_ATTACH_NO_CONNECTOR

When external bridges are attached with DRM_BRIDGE_ATTACH_NO_CONNECTOR,
the panel bridge may also get the same flag, but in the .attach()
callback for the panel bridge a device link is added only when this
flag is not present; To make things worse, the .detach() callback
tries to delete the device link unconditionally and without checking
if it was created in the first place, crashing the kernel with a NULL
pointer kernel panic upon calling panel_bridge_detach().

Fix that by moving the device_link_add() call before checking if the
DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is present.

Fixes: 199cf07 ("drm/bridge: panel: Add a device link between drm device and panel device")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920082727.57729-1-angelogioacchino.delregno@collabora.com

Author: AngeloGioacchino Del Regno

drivers/gpu/drm/bridge/panel.c

@@ -67,14 +67,6 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
 	struct drm_device *drm_dev = bridge->dev;
 	int ret;
 
-	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
-		return 0;
-
-	if (!bridge->encoder) {
-		DRM_ERROR("Missing encoder\n");
-		return -ENODEV;
-	}
-
 	panel_bridge->link = device_link_add(drm_dev->dev, panel->dev,
 					     DL_FLAG_STATELESS);
 	if (!panel_bridge->link) {
@@ -83,6 +75,15 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
 		return -EINVAL;
 	}
 
+	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
+		return 0;
+
+	if (!bridge->encoder) {
+		DRM_ERROR("Missing encoder\n");
+		device_link_del(panel_bridge->link);
+		return -ENODEV;
+	}
+
 	drm_connector_helper_add(connector,
 				 &panel_bridge_connector_helper_funcs);