drm/nouveau: Drop mutex_lock_nested for atomic

Purely conjecture, but I think the original locking inversion with the
legacy page flip code between flipping and ttm's bo move function
shoudn't exist anymore with atomic: With atomic the bo pinning and
actual modeset commit is completely separated in the code patsh.

This annotation was originally added in

commit 060810d7
Author: Ben Skeggs <bskeggs@redhat.com>
Date:   Mon Jul 8 14:15:51 2013 +1000

    drm/nouveau: fix locking issues in page flipping paths

due to

commit b580c9e2
Author: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Date:   Thu Jun 27 13:48:18 2013 +0200

    drm/nouveau: make flipping lockdep safe
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: nouveau@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201127163528.2221671-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -774,7 +774,10 @@ nouveau_bo_move_m2mf(struct ttm_buffer_object *bo, int evict,
 			return ret;
 	}
 
-	mutex_lock_nested(&cli->mutex, SINGLE_DEPTH_NESTING);
+	if (drm_drv_uses_atomic_modeset(drm->dev))
+		mutex_lock(&cli->mutex);
+	else
+		mutex_lock_nested(&cli->mutex, SINGLE_DEPTH_NESTING);
 	ret = nouveau_fence_sync(nouveau_bo(bo), chan, true, ctx->interruptible);
 	if (ret == 0) {
 		ret = drm->ttm.move(chan, bo, &bo->mem, new_reg);