drm/nouveau/fifo/gk104: kick channels when deactivating them

Kicking channels is part of their deactivation process. Maxwell chips
are particularly sensitive to this, and can start fetching the previous
pushbuffer of a recycled channel if this is not done.

While we are at it, improve the channel preemption code to only wait for
bit 20 of 0x002634 to turn to 0, as it is the bit indicating a
preempt is pending.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>