As there is a delay before we pin a vma, there is an opportunity for
another thread to have closed the vm and its vma (including us).
Check as soon as we acquire the vm->mutex and know the vm/vma is stable.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1291


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/20200221121940.2741563-1-chris@chris-wilson.co.uk