drm/i915: capture the correct cursor registers on IVB

This solves some "unclaimed register" messages when there's a GPU hang
on Haswell.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add missing IS_VLV check as spotted by Ville Syrjälä.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>