drm/i915/skl: Skylake moves AUX_CTL from PCH to CPU

So we can apply the old aux_ctl = dp_ctl + 0x10 rule again.

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>