drm/i915/selftests: Also wait for the scratch buffer to be bound

Since PIN_GLOBAL is no longer guaranteed to be synchronous, we must not
forget to include a wait-for-vma prior to execution.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200131142610.3100998-1-chris@chris-wilson.co.uk