drm/tegra: Fix reference leak in tegra_dsi_ganged_probe

The reference taken by 'of_find_device_by_node()' must be released when
not needed anymore. Add put_device() call to fix this.

Fixes: e94236cd ("drm/tegra: dsi: Add ganged mode support")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>