drm/tegra: dpaux: Fix error handling

The devm_pinctrl_register() function returns an error pointer or a valid
handle. So checking for NULL here is pointless and can never trigger.

Check the returned value with IS_ERR instead and propagate this value as
done in the other functions which call devm_pinctrl_register().

Fixes: 0751bb5c ("drm/tegra: dpaux: Add pinctrl support")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>