Move WaEnableYV12BugFixInHalfSliceChicken7 to gen9_init_workarounds

v2: Add stepping check.
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>