drm/i915: Enable HDMI audio for monitor with audio support

Rely on monitor's audio capability to turn on audio output for HDMI.

Tested-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>