drm/nouveau/nvkm: bump maximum number of NVDEC

RM (and GH100) support 8 NVDEC instances.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918202149.4343-3-skeggsb@gmail.com