drm/i915: Use crtc->state for duplication.

In async mode crtc->config can be updated after the locks are released,
resulting in the wrong state being duplicated.

Note that this also removes a spurious assignment of crtc_state->crtc
introduced in

commit f0c60574
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Tue Apr 21 17:12:58 2015 +0300

    drm/i915: Call drm helpers when duplicating crtc and plane states

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>