drm/i915: show unknown sdvox registers on hdmi init

This will throw a BUG() message when an unknown sdvox register is
given to intel_hdmi_init. When this happens, things could going to be pretty
much broken afterwards, so we better detect this as soon as possible.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>