drm/i915: Cursor registers need an offset on VLV

CURSIZE is not present on VLV, so it was left out, as were the IVB
specific cursor B registers.

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