drm/atomic: Convert get_existing_state callers to get_old/new_state, v4.

drm/atomic: Convert get_existing_state callers to get_old/new_state, v4.

This is a straightforward conversion that converts all the users of
get_existing_state in atomic core to use get_old_state or get_new_state

Changes since v1:
- Fix using the wrong state in drm_atomic_helper_update_legacy_modeset_state.
Changes since v2:
- Use the correct state in disable_outputs()
Changes since v3:
- Rebase for link status training.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/df91a9f9-005e-bcbd-1f74-03c38e1e21dd@linux.intel.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>