drm/i915: Whitelist COMMON_SLICE_CHICKEN2

This allows userspace to use "legacy" mode for push constants, where
they are committed at 3DPRIMITIVE or flush time, rather than being
committed at 3DSTATE_BINDING_TABLE_POINTERS_XS time.  Gen6-8 and Gen11
both use the "legacy" behavior - only Gen9 works in the "new" way.

Conflating push constants with binding tables is painful for userspace,
we would like to be able to avoid doing so.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911014801.26821-1-kenneth@whitecape.org