drm/i915: Add support for audio driver notifications

Notifiations like mode change, hot plug and edid to
the audio driver are added. This is inturn used by the
audio driver for its functionality.

A new interface file capturing the notifications needed by the
audio driver is added

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Jerome Anand <jerome.anand@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>