drm/i915: Add support for VGA load detection (pre-945).

Two approaches for VGA detections: hot plug detection for 945G onwards
and load pipe detection for Pre-945G.  Load pipe detection will get one free
pipe, set border color as red and blue, then check CRT status by
swf register.  This is a sync-up with the 2D driver.

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>