drm/i915/selftests: Magic numbers for old Y-tiling

i915g has a slightly different tiling layout, and so requires a
different reference swizzle pattern.

Testcase: igt/drv_selftests/live_objects #gdg
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180707100405.817-1-chris@chris-wilson.co.uk