drm/i915: Use drm_atomic_helper_update_legacy_modeset_state, v2.

Now that the helper is exported there's no need to duplicate
this code any more.

Changes since v1:
- move intel_modeset_update_staged_output_state call to the right place.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>