drm/i915: move swap state to the right place

This is a preparation for passing crtc state to the helpers.
When converting all users of crtc->config to use the old or
new state it's easier to find regressions when swap_state is
done first.

If crtc->config is swapped at the same place as swap_state
bugs will never be found.

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>