drm/tegra: Add missing clk_disable_unprepare() in tegra_dc_probe()

Add the missing clk_disable_unprepare() before return from
tegra_dc_probe() in the error handling path.

Fixes: f68ba691 ("drm/tegra: dc: Link DC1 to DC0 on Tegra20")
Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>