drm/exynos: dsi: do not try to find bridge

It doesn't need to try to find a bridge if bridge node doesn't exist.

Reviewed-by: Shuah Khan <shuahkh@osg.samsung.com>
Tested-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>