drm/tegra: sor: Move register programming out of ->init()

The hardware is not guaranteed to be enabled during execution of the
tegra_sor_init() function, which can lead to a crash on some Tegra SoCs.
Fix this by moving all register programming into code that is guaranteed
to only be executed when the hardware is enabled.

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