drm/i915: Be defensive when starting vma activity

Before we acquire the vma for GPU activity, ensure that the underlying
object is not already in the process of being freed.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820100531.8430-1-chris@chris-wilson.co.uk