drm/i915: Add crtc state duplication/destruction functions

The atomic helpers need these to prepare a new state object when
starting a new atomic operation.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>