drm/i915/selftests: Add the usual batch vma managements to st_workarounds

To properly handle asynchronous migration of batch objects, we need to
couple the fences on the incoming batch into the request and should not
assume that they always start idle.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190826072149.9447-1-chris@chris-wilson.co.uk