plugins: expose I420 format for interop with SW elements.

plugins: expose I420 format for interop with SW elements.

Always expose I420 format by default when the VA surface could be
mapped for interoperability with non harware accelerated elements.
However, the default behaviour remains the auto-plugging of vaapi
elements, down to the sink.

Side effect: "direct-rendering" mode is also disabled most of the
times as plain memcpy() from uncached speculative write combining
memory is not going to be efficient enough.