drm/nvd0/disp: mask off high 16 bit of negative cursor x-coordinate

commit af5e7d84b0ec45b2b614b0d6e3657cbdceaa21f9 upstream.

Signed-off-by: Christoph Bumiller <e0425955@student.tuwien.ac.at>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>