vpp: fix check for non-linear anamorphic scaling on Sandybridge.

Fix pp_nv12_avs_initialize() to disable non-linear anamorphic scaling
when not actually requested on Sandybridge. This is now the main entry
point for advanced video scaling there, so the actual scaling mode is
checked for beforehand.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>