drm/i915/selftests: Serialise write to scratch with its vma binding

Add the missing serialisation on the request for a write into a vma to
wait until that vma is bound before being executed by the GPU.

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/20191011193620.14026-1-chris@chris-wilson.co.uk