drm/i915: HDMI - Clear Audio Enable bit for Hot Plug

Clear Audio Enable bit to trigger unsolicated event to notify Audio
Driver part the HDMI hot plug change. The patch fixed the bug when
remove HDMI cable the bit was not cleared correctly.

In intel_hdmi_dpms(), if intel_hdmi->has_audio been true, the "Audio enable bit" will
be set to trigger unsolicated event to notify Alsa driver the change.

intel_hdmi->has_audio will be reset to false from intel_hdmi_detect() after
remove the hdmi cable, here's debug log:

[  187.494153] [drm:output_poll_execute], [CONNECTOR:17:HDMI-A-1] status updated from 1 to 2
[  187.525349] [drm:intel_hdmi_detect], HDMI: has_audio = 0

so when comes back to intel_hdmi_dpms(), the "Audio enable bit" will not be cleared. And this
cause the eld infomation and pin presence doesnot update accordingly in alsa driver side.

This patch will also trigger unsolicated event to alsa driver to notify the hot plug event:

[  187.853159] ALSA sound/pci/hda/patch_hdmi.c:772 HDMI hot plug event: Codec=3 Pin=5 Presence_Detect=0 ELD_Valid=1
[  187.853268] ALSA sound/pci/hda/patch_hdmi.c:990 HDMI status: Codec=3 Pin=5 Presence_Detect=0 ELD_Valid=0

Signed-off-by: Wang Xingchao <xingchao.wang@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>