Revert "drm/i915/sprite: Always enable the scaler on IronLake"

Revert "drm/i915/sprite: Always enable the scaler on IronLake"

Apparently always enabling the sprite scaler magically made
sprites work on ILK in the past.

I think the real reason for the failure was missing sprite
watermark programming, and enabling the scaler effectively
disabled LP1+ watermarks, which was enough to keep things going.
Or it might be that the hardware more or less ignores watermarks
for scaled sprites since things seem to work even if I leave
sprite watermarks at 0 and disable all other planes except the
sprite.

In any case, we left the scaler always on but then failed to
check whether we might be exceeding the scaler's source size
limits. That caused the sprite to fail when a sufficiently
large unscaled image was being displayed.

Now that we're getting proper watermark programming for ILK, we
can keep the scaler disabled unless we need to do actual scaling.

This reverts commit 8aaa81a166d80ac9bf2813984e5b4c2503d0fe08.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>