modetest: Add YUV420 support and fix YVU420 Cb/Cr ordering

YUV420 support is trivial to add since the code already supports
YVU420.

But it looks like the YVU420 support is a bit broken. The chroma
planes are passed in the wrong order to the fill functions, so
fix that while were at it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>