drm/i915: Fix HDMI mode select for Cougarpoint PCH

For real HDMI sink, CPT HDMI port has to set 'HDMI' mode flag
in order to make HDMI audio work correctly.

This is required patch for drm/i915 to enable HDMI audio on CPT PCH,
ALSA patch is at http://mailman.alsa-project.org/pipermail/alsa-devel/2010-May/027601.html

Tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>