drm/i915: Fix VLV sprite register offsets

We forgot to add VLV_DISPLAY_BASE to the VLV sprite registers, which
caused the sprites to not work at all.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>