drm/i915/hdmi: fix hdmi audio state readout

Check the correct bit for audio. Seems like a copy-paste error from the
start:

commit 9ed109a7


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Apr 24 23:54:52 2014 +0200

    drm/i915: Track has_audio in the pipe config

Reported-by: Martin Andersen <martin.x.andersen@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82756
Cc: stable@vger.kernel.org # 3.16+
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>