Simplify scan mode selection.
Simplify scan mode selection.

- The scan mode selection for chroma was a bit complicated so I checked it
  and it was all unnecessary. The mode selection is the same as for luma.

ultravideo/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar