drm/i915: add DisplayPort amp unmute for LPE audio mode

Enable unmute/mute amp notification. This doesn't seem to affect
HDMI support so this is done unconditionally.

An earlier version of this patch set a chicken bit at address 0x62F38
prior to the mute/unmute but this register doesn't seem to do anything
so this phase was removed.

v1->v2: Drop needless pipe A check, avoid temporary reg offset variable.
v2->v3: Add "_" prefix to VLV_AUD_PORT_EN_X_DBG as they are internal.

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