drm/i915/sprite: Avoid incurring extra vblank stall when updating plane on IVB

IvyBridge requires an extra frame between disabling the low power
watermarks and enabling scaling on the sprite plane. If the scaling
is already enabled, then we have already disabled the low power
watermarks and need not incur an extra wait.

Similarly, as we disable the scaling when turning off the sprite plane,
we can update the scaling enabled flag and restore the low power
watermarks.

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>