drm/i915: DisplayPort hot remove notification to audio driver

On DP monitor hot remove, clear DP_AUDIO_OUTPUT_ENABLE accordingly,
so that the audio driver will receive hot plug events and take action
to refresh its device state and ELD contents.

Note that the DP_AUDIO_OUTPUT_ENABLE bit may be enabled or disabled
only when the link training is complete and set to "Normal".

Tested OK for both hot plug/remove and DPMS on/off.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>