Change CUDA stereo to keep disparity type when not empty.

git commits d0e89337da2e7fe01dbaa249ad262b36c1463f45 ("refactored
StereoBeliefPropagation") and be9bb8f18b781ef84bf69b9462642b86ab067ff0
("refactored StereoConstantSpaceBP") changed the documented behavior
for these algorithms, turning the outputs into CV_16SC1 unless it was
actually impossible to do so.  As a result, the sample stereo_match code
displayed a uselesss blank gray image.