drm/i915: Disable SAMPLER_STATE prefetching on all Gen11 steppings.

The Demand Prefetch workaround (binding table prefetching) only applies
to Icelake A0/B0.  But the Sampler Prefetch workaround needs to be
applied to all Gen11 steppings, according to a programming note in the
SARCHKMD documentation.

Using the Intel Gallium driver, I have seen intermittent failures in
the dEQP-GLES31.functional.copy_image.non_compressed.* tests.  After
applying this workaround, the tests reliably pass.

v2: Remove the overlap with a pre-production w/a

BSpec: 9663
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625090655.19220-1-chris@chris-wilson.co.uk