drm/tegra: dsi: Add missing check for of_find_device_by_node

Add check for the return value of of_find_device_by_node() and return
the error if it fails in order to avoid NULL pointer dereference.

Fixes: e94236cd ("drm/tegra: dsi: Add ganged mode support")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231024080738.825553-1-nichen@iscas.ac.cn