drm/i915: Bump object bookkeeping to u64 from size_t

Internally we allow for using more objects than a single process can
allocate, i.e. we allow for a 64bit GPU address space even on a 32bit
system. Using size_t may oveerflow.

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/20161018120251.25043-1-chris@chris-wilson.co.uk