drm/bridge: it66121: Wait for next bridge to be probed

If run before the next bridge is initialized, of_drm_find_bridge() will
give us a NULL pointer.

If that's the case, return -EPROBE_DEFER; we may have more luck next
time.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Fixes: 988156dc ("drm: bridge: add it66121 driver")
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210827163956.27517-2-paul@crapouillou.net