drm/i915/sdvo: Extract intel_sdvo_has_audio()

Pull the SDVO audio state computation into a helper.

This is almost identical to intel_hdmi_has_audio(),
except the sink capabilities are stored under intel_sdvo
rather than intel_hdmi. Might be nice to get rid of
this duplication eventually...

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
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/20221026170150.2654-16-ville.syrjala@linux.intel.com