drm: bridge/panel: Cleanup connector on bridge detach

In the Linux kernel, the following vulnerability has been resolved:

drm: bridge/panel: Cleanup connector on bridge detach

If we don't call drm_connector_cleanup() manually in

panel_bridge_detach(), the connector will be cleaned up with the other

DRM objects in the call to drm_mode_config_cleanup(). However, since our

drm_connector is devm-allocated, by the time drm_mode_config_cleanup()

will be called, our connector will be long gone. Therefore, the

connector must be cleaned up when the bridge is detached to avoid

use-after-free conditions.

v2: Cleanup connector only if it was created

v3: Add FIXME

v4: (Use connector->dev) directly in if() block