drm/nouveau/disp/nv50-gp10x: fix coverity warning

Change values to u32, there's no need for them to be 64-bit.

Reported-by: Colin Ian King <colin.king@canonical.com>
Suggested-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>