examples: use I420 input for encoders

The output of simple_decoder, as well as the default format for the
ivf{enc,dec} tools, is I420. The encoder examples are changed to default
to I420 as well. This is desirable because passing the output of
simple_encoder to simple_decoder would otherwise have the chroma planes
flipped, which is unexpected.

Change-Id: I9d4f10ace594a54a7fc4f9a1f9187365c01fd638