drm/i915: Limit the scattergather coalescing to 32bits

The scattergather list uses a 32bit size counter, we should avoid
exceeding it.

v2: Also we should use unsigned int to match sg->length.

Fixes: 871dfbd6 ("drm/i915: Allow compaction upto SWIOTLB max segment size")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161018120251.25043-3-chris@chris-wilson.co.uk