drm/tegra: dc: Move more code into ->init()

The code in tegra_crtc_prepare() really belongs in tegra_dc_init(), or
at least most of it. This fixes an issue with VBLANK handling because
tegra_crtc_prepare() would overwrite the interrupt mask register that
tegra_crtc_enable_vblank() had written to to enable VBLANK interrupts.

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