BDW+: disable SGVS

BDW+: disable SGVS

BDW introduces separate packets for controlling instancing and system
generated values (eg vertex id and instance id).  We don't use
instancing, but still need to disable sgvs to avoid undefined behaviour
when some other driver (mesa) uses it.

Signed-off-by: Joe Konno <joe.konno@intel.com>
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Daniel Charles <daniel.charles@intel.com>
(cherry picked from commit 4caa37b895d39af754e2b2307879752130731f56)