drm/exynos: remove in_bridge_node from exynos_dsi

We do not need to keep a reference to the in_bridge_node, but we can
simply drop it, once we found and attached the previous bridge.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>