drm/i915: Fix HSW parity test

Haswell changed the log registers to be WO, so we can no longer read
them to determine the programming (which sucks, see later note). For
now, simply use the cached value, and hope HW doesn't screw us over.

v2: Simplify the logic to avoid an extra !, remove last, and fix the
buffer offset which broke along the rebase (Ville)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57441


CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>