drm/i915: Make intel_dp->has_audio reflect hw state only

Always detect if audio is available during edid detection.
With less magic switching it's easier to convert the dp connector
properties to atomic.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170501133804.8116-9-maarten.lankhorst@linux.intel.com