drm/i915: overlay: nuke readback to flush wc caches

I retested this and whatever this papered over, the problem doesn't seem
to exist anymore.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
[anholt: fixed up compile warning]
Signed-off-by: Eric Anholt <eric@anholt.net>