drm/i915/hdmi: Enable hdmi mode on g4x, too

For compliance we really should be sending NULL infoframes always
when we detect a hdmi capable monitor. Also remove the now redudant
setting for the has_audio case and enforce that audio is only
possible with a hdmi sink.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>