drm/i915: Disable SF pipelined attribute fetch for SNB

According to Bspec we need to disable SF pipelined attribute fetch
whenever SF outputs exceed 16 and normal clip mode is used. A quick
glance at Mesa suggests that these conditions could happen. So let's
just always set the magic bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>