drm/tegra: sor: Reset during initialization

As there isn't a way for the firmware on the Nyan Chromebooks to hand
over the display to the kernel, and the kernel isn't redoing the whole
configuration at present.

With this patch, the SOR is brought to a known state and we get correct
display on every boot.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>