drm/i915: BDW can also detect unclaimed registers

By the time I wrote this patch, it allowed me to catch some problems.
But due to patch reordering - in order to prevent fake "regression"
reports - this patch may be merged after the fixes of the problems
identified by this patch.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>