drm/nouveau/ttm: always do buffer moves on kernel channel

There was once good reasons for wanting the drm to be able to use M2MF etc
on user channels, but they're not relevant anymore.  For the general
buffer move case, we've already lost by transferring between vram/sysmem
already so the context switching overhead is minimal in comparison.

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