Comment 0 for bug 1738334

ea642c3216cb ("drm/ttm: add io_mem_pfn callback") is incomplete: the hisilicon hibmc driver does not contain the callback and so X does not start.

Discussion and potential fix at https://lists.freedesktop.org/archives/dri-devel/2017-November/159002.html

This affects Artful, upstream has not landed on a solution yet as far as I can tell, so lets backport the first proposed small fix.