Change default behavior to assume sampled chroma

When calling check_initial_width through vp9_set_size_literal
the function was defaulting to using non-subsampled chroma.

This patch changes the default to assume sampled chroma as
an interim solution until complete support for other
color formats is added.

Change-Id: Id8e7e919b350e3473dfdf7551af6fd0716478b04