drm/i915: Don't populate plane->i9xx_plane for sprites

enum i9xx_plane_id namespace is not valid for any sprite plane,
so let's not even populate plane->i9xx_plane.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-4-ville.syrjala@linux.intel.com