drm/vmwgfx: Drop authentication requirement on UNREF ioctls

These ioctls will anyway only succeed if the client previously opened
referenced the object. Furthermore, closing the client would implicitly
execute the same action. This prevents clients from blocking on UNREF if
their master dropped, and will allow masters to UNREF after dropping
master privileges.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>