drm/nouveau: Evict buffers in VRAM before freeing sgdma

Currently, we take down the sgdma engine without evicting all buffers
from VRAM.

The TTM device release will try to evict anything in VRAM to GART
memory, but this will fail since sgdma has already been taken down.

This causes an infinite loop in kernel mode on module unload.
It usually doesn't happen because there aren't any buffer on close.
However, if the GPU is locked up, this condition is easily triggered.

This patch fixes it in the simplest way possible by cleaning VRAM
right before cleaning SGDMA memory.

Signed-off-by: Luca Barbieri <luca@luca-barbieri.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>