drm/tegra: dc: Reset VBLANK to off

Upon driver load, reset the VBLANK machinery to off to reflect the
hardware state. Since the ->reset() callback is called from the initial
drm_mode_config_reset() call, move the latter after the VBLANK machinery
initialization by drm_vblank_init().

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