drm/i915: Removed IVB forced enable of sprite dest key.

The destination color key is always enabled for IVB.  Removed
the line that does this.

Signed-off-by: Armin Reese <armin.c.reese@intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>