The mfx implementation based on D3D11 is expected for an internal
session on Windows, however sometimes this implemntation is not
supported [1]. A fallback to the default mfx implementation is added in
this patch.
[1] https://github.com/intel/cartwheel-ffmpeg/issues/246
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>