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

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 f5e7840b0c4368f8cdbb055188c2a0eef50c3052
Author: Thierry Reding <[email protected]>
Date:   Wed Jan 28 14:54:32 2015 +0100

    drm/atomic: Add helpers for state-subclassing drivers

Signed-off-by: Ander Conselvan de Oliveira <[email protected]>
Reviewed-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>