drm/i915: Support IGD EOS

In the event that any one of the DAC analog outputs (R,G,B) were driven
at full-scale (white video) or some analog level close to full-scale
voltage, and if the video cable were then disconnected, the analog video
voltage level would exceed the maximum electrical overstress limit of the
native (thin-oxide) transistors thus causing a long-term reliability concern.
The electrical overstress condition occurs in this particular case.

This patch address the IGD EOS (electrical overstress condition) issue.
When the EOS interrupt occurs, OS should disable DAC and then disable EOS,
then the normal hotplug operation follows.

TODO: it appears the normal unplug interrupt is missed as reported by Li Peng,
need more checks here.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>