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

Use the helpers introduced by the commit below to properly initialize
the duplicated states.

commit f5e7840b
Author: Thierry Reding <treding@nvidia.com>
Date:   Wed Jan 28 14:54:32 2015 +0100

    drm/atomic: Add helpers for state-subclassing drivers

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>