anv/allocator: Misc cleanups for BOs
Getting a few patches out of a project branch. I think these generally make the driver better so we may as well land them now. The core objective here is to keep our i915 interactions contained instead of misc. places all over the driver calling misc. ioctls. execbuf is pretty contained and, once !13427 (merged) lands, synchronization will be too. Right now, though, we poke at anv_bo::gem_handle
and call ioctls all over the place.