drm/i915: Fix comparison bug

drm/i915: Fix comparison bug

->stolen->start has type u64 aka unsigned long long; relying on the
difference (effectively cast to int) for sorting is wrong.

It wouldn't be a problem in practice if the values compared are always
within INT_MAX of each other (so that the difference is actually
representable in an int), but 440fd5283a87 ("drm/mm: Support 4 GiB and
larger ranges") strongly suggests that's not the case.

Note: atm we don't support more than about 1G of stolen, so this is
impossible currenlty.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
[danvet: Add note that this is impossible currently.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>