drm/i915: HDMI hot remove notification to audio driver

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

The cleared SDVO_AUDIO_ENABLE bit needs to be restored to prevent losing
HDMI audio after DPMS on.

CC: Wang Zhenyu <zhenyu.z.wang@intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>