drm/i915: hdmi detection according by reading edid

According to investigations from windows team ,hw team,
and our test results on all 4x platofrms available
(gm45, g45b, q45, g45a, g45c, g41a, and g41), we find
currently Hot plug live status and Hot plug interrupt
detection are not reliable, sometime the results from
the two approaches are contradicts. So we chose edid
detection for hdmi output.

Signed-off-by: Ma Ling <ling.ma@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>