drm/i915: Double check we didn't miss an unclaimed register access

Currently, if the user has enabled mmio-debug around each register
access, we presume that we have then checked them all. However, it is
still possible through omission (raw register access) or external
interaction that the unclaimed access was not highlighted.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904111732.24266-1-chris@chris-wilson.co.uk