drm/nouveau: Don't take dev->struct_mutex in ttm_fini

This is only called in driver load/unload paths, no need to grab any
locks at all. Also, ttm takes care of itself anyway.

Cc: Ben Skeggs <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/nouveau/nouveau_ttm.c

···
 void
 nouveau_ttm_fini(struct nouveau_drm *drm)
 {
-	mutex_lock(&drm->dev->struct_mutex);
 	ttm_bo_clean_mm(&drm->ttm.bdev, TTM_PL_VRAM);
 	ttm_bo_clean_mm(&drm->ttm.bdev, TTM_PL_TT);
-	mutex_unlock(&drm->dev->struct_mutex);
 
 	ttm_bo_device_release(&drm->ttm.bdev);