drm/tegra: sor: Use correct SOR index on Tegra210

The device tree bindings for the Tegra210 SOR don't require the
controller instance to be defined, since the instance can be derived
from the compatible string. The index is never used on Tegra210, so we
got away with it not getting set. However, subsequent patches will
change that, so make sure the proper index is used.

Signed-off-by: Thierry Reding <treding@nvidia.com>