drm/i915: move atomic_helper under display sub-struct

Move display atomic helper related members under drm_i915_private
display sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1d864238a92a32d52ea70c0079c910cc90955324.1661779055.git.jani.nikula@intel.com