drm/i915: Allow dead vm to unbind vma's without lock.

i915_gem_vm_close may take the lock, and we currently have no better way
of handling this. At least for now, allow a path in which holding vm->mutex
is sufficient. This is the case, because the object destroy path will
forcefully take vm->mutex now.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128085739.1464568-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>