sna/video/sprite: Make NV12 take the GPU scaling fallback

Even unscaled NV12 needs the plane scaler on SKL+, so when
unscaled NV12 setplane fails we should still take the GPU
scaling fallback path.

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