drm/vmwgfx: Fix a case where the code would BUG when trying to pin GMR memory

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Dmitry Torokhov <dtor@vmware.com>
Cc: stable@vger.kernel.org
Cc: linux-graphics-maintainer@vmware.com
Signed-off-by: Dave Airlie <airlied@redhat.com>