drm/i915: Reorder the AUX_CTL bits in descending order

So it's easier to compare what we program with the documentation, not
having to jump at all.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>