drm/i915/selftests: Mark up write into scratch vma

We correctly attach the exclusive fetch for the scratch object when
emitting a request that writes into it, but for completeness we should
also declared the write to i915_vma_move_to_active()

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180629133717.11761-1-chris@chris-wilson.co.uk