drm/bridge: Fix unlocked list_del in drm_bridge_add()

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

drm/bridge: Fix unlocked list_del in drm_bridge_add()

When re-adding a bridge that was previously removed, drm_bridge_add()
drops it from bridge_lingering_list without holding bridge_lock.

Both bridge_list and bridge_lingering_list are protected by bridge_lock,
as they are concurrently modified by drm_bridge_remove() and
__drm_bridge_free(), and walked by the debugfs 'bridges' file. Running
the list_empty() test and the list_del_init() outside of the lock may
therefore corrupt either list.

Perform both under bridge_lock.