drm/i915: Set crtc backpointer when duplicating crtc state

In the path were there is no state to duplicate, the allocated crtc
state wouldn't have the crtc backpointer initialized.

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