drm/i915: Set crtc->new_config to NULL for pipes that are about to be disabled

crtc->new_config is only relevant for pipes that are going to be active
post-modeset. Set the pointer to NULL for all pipes that are going to
be disabled. This is done to help catch bugs where some piece of code
would go looking at crtc->new_config even if the data there is stale.

v2: Clear new_config in disable_crtc_nofb() too (Imre)

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>