drm/bridge: ps8640: Get the EDID from eDP control

The PS8640 DSI-to-eDP bridge can retrieve the EDID, so implement the
.get_edid callback and set the flag to indicate the core to use it.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826081526.674866-3-enric.balletbo@collabora.com