drm/i915: Enable VLV audio chicken bit for LPE audio

The audio chicken bit (register offset 0x62f38) seems required to make
DP audio working on some machines.  At least, on Dell Wyse 3040, I
failed to get the audio unless this bit is set once.

Strangely, the bit seems necessary only once, and it persists after
that, even some power-off cycles.  The register is supposedly
write-only, so it's no evidence whether the bit keeps effect
persistently.  But, judging from the experiment, it looks enough to
set it up once at the device initialization.

The patch is basically a cut from the original patch by Pierre-Louis
Bossart.

v1->v2: drop read since it's a write-only reg.

Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>