drm/i915: Use fixed-sized types for stolen

Stolen memory is a hardware resource of known size, so use an accurate
fixed integer type rather than the ambiguous variable size_t. This was
motivated by the next patch spotting inconsistencies in our types.

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/20170106152013.24684-3-chris@chris-wilson.co.uk