drm/bridge: Fix lvds-encoder since the panel_bridge rework.

The panel_bridge bridge attaches to the panel's OF node, not the
lvds-encoder's node.  Put in a little no-op bridge of our own so that
our consumers can still find a bridge where they expect.

This also fixes an unintended unregistration and leak of the
panel-bridge on module remove.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 13dfc054

 ("drm/bridge: Refactor out the panel wrapper from the lvds-encoder bri
dge.")
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114191647.22207-1-eric@anholt.net