drm/exynos/dsi: fix bridge_node DT parsing

DSIM uses MIC bridge which is between DECON and DSIM, so the driver
should expect bridge node on input side.

Fixes: 86418f90 ("drm: convert drivers to use of_graph_get_remote_node")
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>