drm/i915: enable HDMI mode on VLV when an HDMI sink is detected

Allows sending of the null packets for conformance.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>