drm/i915: Disable DP audio for g4x

Apparently g4x doesn't support audio over DP. Bspec lists the
bit as "Reserved for Audio Output Enable", and empirical evidence
tells us that the bit won't stick. So stop trying to enable DP
audio on g4x.

Link: https://patchwork.freedesktop.org/patch/msgid/20171129164303.18793-1-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103989
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>