drm/tegra: sor: Write correct head state registers

drm/tegra: sor: Write correct head state registers

The head state registers are per head, so they must be properly indexed.
This has worked fine so far because all boards with eDP use it as the
primary output, so it is very likely to end up attached to head 0.

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