mesa/st: Make st_pipe_format_to_mesa_format an effective no-op.

All callers other than the unit test just wanted to convert back from
a known-mesa-equivalent format, which is now a no-op.

v2: Fix assertion failure in iris GL startup with BGR565 by continuing
    to return MESA_FORMAT_NONE for non-Mesa formats.

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)