anv: Require softpin on Gen8+
Softpin was added to i915 in
commit 506a8e87d8d2746b9e9d2433503fe237c54e4750 Author: Chris Wilson <firstname.lastname@example.org> Date: Tue Dec 8 11:55:07 2015 +0000 drm/i915: Add soft-pinning API for execbuffer
which was included in Linux 4.5. It's been over 4 years so it's probably reasonable to make it a hard requirement.