drm/nouveau/fb/tu102-: fix register used to determine scrub status

Turing apparently needs to use the same register we use on Ampere.

Not executing the scrubber ucode when required would result in large
areas of VRAM being inaccessible to the driver.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230130223715.1831509-2-bskeggs@redhat.com