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

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43020
Reported-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>