drm/i915: Use new atomic iterator macros in display code

Add a big fat warning in __intel_display_resume that the old state is
invalid, and use the correct state everywhere.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1489071125-917-5-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[mlankhorst: Change one occurence of conn_state to new_conn_state in
             verify_connector_state, and drop old_conn_state there]