drm/i915: Fixup cursor latency used for IVB lp3 watermarks

It operates at twice the declared latency, so adjust the computation to
avoid potential flicker at low power.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50248


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