drm/bridge: lt8912b: return EPROBE_DEFER if bridge is not found

Returns EPROBE_DEFER when of_drm_find_bridge() fails, this is consistent
with what all the other DRM bridge drivers are doing and this is
required since the bridge might not be there when the driver is probed
and this should not be a fatal failure.

Cc: <stable@vger.kernel.org>
Fixes: 30e2ae94

 ("drm/bridge: Introduce LT8912B DSI to HDMI bridge")
Signed-off-by: Matheus Castello <matheus.castello@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230322143821.109744-1-francesco@dolcini.it