drm/bridge: nxp-ptn3460: add drm_panel_bridge support

drm/bridge: nxp-ptn3460: add drm_panel_bridge support

Prepare the bridge driver for use in a chained setup.

- Replacing direct use of drm_panel with drm_panel_bridge support.
- Make the connector creation optional

Note: the bridge panel will use the connector type from the panel.

v3:
  - Fix wrong logic in connector creation (Laurent)

v2:
  - Use panel_bridge for local variable name to align with other drivers
  - Fix that connector was created twice (Laurent)
  - Set bridge.type to DRM_MODE_CONNECTOR_LVDS.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727170320.959777-6-sam@ravnborg.org