drm/nouveau/gr/gm107-: fix touching non-existent ppcs in attrib cb setup

Also removes an XXX; according to nvgpu headers the field is called
NV_PGRAPH_GPCS_SWDX_TC_BETA_CB_SIZE_DIV3, so, apparently not some
magic we need to figure out :)

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>