drm/imx-ldb: Add support to drm-bridge

Add support to attach a drm_bridge to imx-ldb in addition to
existing support to attach a LVDS panel.

This patch does a simple code refactoring by moving code
from for_each_child_of_node iterator to a new function named
imx_ldb_panel_ddc(). This was necessary to allow the panel ddc
code to run only when the imx_ldb is not attached to a bridge.

Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>