drm/i915: Do no set Stencil Cache eviction LRA w/a on gen7+

I've flagged this while reviewing the first version and Ken Graunke
fixed it up in v2, but unfortunately Dave Airlie picked up the wrong
version.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: stable@kernel.org
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>