drm/cirrus: Invalidate page tables when pinning a BO

This is a cirrus version of Egbert Eich's patch for mgag200.

Without bo.bdev->dev_mapping set, the ttm_bo_unmap_virtual_locked
called from ttm_bo_handle_move_mem returns with no effect. If any
application accessed the memory before it was moved, it will
access wrong memory next time. This causes crashes when changing
resolution down.
Signed-off-by: Michal Srb <msrb@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/cirrus/cirrus_ttm.c

@@ -328,6 +328,7 @@ int cirrus_bo_create(struct drm_device *dev, int size, int align,
 
 	cirrusbo->gem.driver_private = NULL;
 	cirrusbo->bo.bdev = &cirrus->ttm.bdev;
+	cirrusbo->bo.bdev->dev_mapping = dev->dev_mapping;
 
 	cirrus_ttm_placement(cirrusbo, TTM_PL_FLAG_VRAM | TTM_PL_FLAG_SYSTEM);