drm/i915: Fix offset page-flips on i965+

i965 uses the Display Registers to compute the offset from the display
base so the new base does not need adjusting when flipping. The older
chipsets use a fence to access the display and so do perceive the
surface as linear and have a single base register which is reprogrammed
using the flip.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-by: Marty Jack <martyj19@comcast.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>