drm/i915: use the new hdmi_force_audio enum more

While fixing up a merge conflict with drm-next I've noticed that we
use the same audio drm connector property also for dp and sdvo
outputs.

So put the new enum to some good use and convert these paths, too. The
HDMI_AUDIO_ prefix is a bit a misnomer. But at least for sdvo it makes
sense (and you can also connect a hdmi monitor with a dp->hdmi cable),
so I've decided to stick with it.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>