drm/i915: BUG_ON bad PPGTT offset

Because PPGTT PDEs within the GTT are calculated in cachelines
(HW guys consistency ftw) we do a divide which will wreak havoc if this
is wrong, and I know that from experience).

If/when we move to multiple PPGTTs this will have to become a WARN, and
return an error. For now however it should always be considered fatal,
and only a developer could hit it.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: s/BUG/WARN]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>