drm/bridge: nxp-ptn3460: add get_edid bridge operation

Add the get_edid() bridge operation to prepare for
use as a chained bridge.
Add helper function that is also used by the connector.

v2:
  - Fix memory leak (Laurent)
  - Do not save a copy of edid, read it when needed

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/20200727170320.959777-5-sam@ravnborg.org