drm/i915/dp: fix DP audio for PORT_A on gen12+

Starting with gen12, PORT_A can be connected to a transcoder
with audio support. Modify the existing logic that disabled
audio on PORT_A unconditionally.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191125125313.17584-1-kai.vehmanen@linux.intel.com