drm/ast: invalidate page tables when pinning a BO

same fix as cirrus and mgag200.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ast/ast_ttm.c

@@ -323,6 +323,7 @@ int ast_bo_create(struct drm_device *dev, int size, int align,
 
 	astbo->gem.driver_private = NULL;
 	astbo->bo.bdev = &ast->ttm.bdev;
+	astbo->bo.bdev->dev_mapping = dev->dev_mapping;
 
 	ast_ttm_placement(astbo, TTM_PL_FLAG_VRAM | TTM_PL_FLAG_SYSTEM);