drm/nouveau: bring back blit subchannel for pre nv50 GPUs

1ba6113a90a0 removed a lot of the kernel GPU channel, but method 0x128
was important as otherwise the GPU spams us with `CACHE_ERROR` messages.

We use the blit subchannel inside our vblank handling, so we should keep
at least this part.

v2: Only do it for NV11+ GPUs

Closes: drm/nouveau#201


Fixes: 4a16dd9d ("drm/nouveau/kms: switch to drm fbdev helpers")
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230526091052.2169044-1-kherbst@redhat.com