anv: Require softpin on Gen8+

Softpin was added to i915 in

    commit 506a8e87d8d2746b9e9d2433503fe237c54e4750
    Author: Chris Wilson <chris@chris-wilson.co.uk>
    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.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
23 jobs for !4431 with review/anv-require-softpin in 9 minutes and 21 seconds (queued for 3 seconds)
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