drm/nouveau: use dedicated channel for async moves on GT/GF chipsets.

The moves themselves were generally async to graphics previously, with
the exception that if the "main" channel is used to synchronise a
page flip at the same time, it can end up blocked for a noticable amount
of time for large buffer moves.

Not really critical, and there's better ways of handling this, but they
are all rather invasive, so this is fine for now.

Based on a patch by Maarten Lankhorst addressing the same issue.

Reported-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>