drm/i915: Stop using obj->obj_exec_link outside of execbuf

i915_gem_stolen_list_info() sneakily takes advantage of the
obj->obj_exec_link to save itself from having to allocate. Enough of the
subterfuge, just allocate an array of pointers and sort them instead of
the list.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170316132006.7976-7-chris@chris-wilson.co.uk