drm/i915: Don't use crtc->plane in ILK+ get_config()

crtc->plane can only be different from crtc->pipe pre-Gen4. Don't use it
in new-ish code.

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>