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

commit afcc87aa6a233e52df73552dc1dc9ae3881b7cc8 upstream.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Dmitry Torokhov <dtor@vmware.com>
Cc: linux-graphics-maintainer@vmware.com
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>