drm/i915/sdvo: Only use "presence detect" for has_audio readout

Align the SDVO audio code with the native HDMI/DP audio and
use just the "presence detect" bit for the has_audio readout.
The "ELD valid" bit will be used for ELD readout soon.

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230124144628.4649-9-ville.syrjala@linux.intel.com