sna/video: Actually use the NV12 shader on gen8

Oops. We never actually select the NV12 shader on gen8, causing
us to render garbage. Looks like this is the only one I somehow
missed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>