drm/bridge: lt8912b: use ->edid_read callback

Prefer using the struct drm_edid based functions.

Cc: Adrien Grassein <adrien.grassein@gmail.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Robert Foss <rfoss@kernel.org>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/98f0356f1bd279bc337c904d8313c9afee530b48.1706038510.git.jani.nikula@intel.com