drm/tegra: Fix gpiod_get_from_of_node() regression

That function now returns ERR_PTR instead of NULL if "hpd-gpio" is not
present in device-tree. The offending patch missed to adapt the Tegra's
DRM driver for the API change.

Fixes: 025bf377

 ("gpio: Fix return value mismatch of function gpiod_get_from_of_node()")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>