drm/i915: Clear FlipDone semantics change for pageflipping on gen3

drm/i915: Clear FlipDone semantics change for pageflipping on gen3

On later gen3, you are able to select the meaning of the FlipPending
status bit in IIR and change it to FlipDone. This was sometimes done by
the BIOS leading to confusion on just how pageflipping worked on gen3.
Simplify the implementation by using the legacy meaning for all gen3
machines.

Note: this makes all gen3 machines equally broken...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>