st/mesa: fix typos and add some format fallbacks in format_map

st/mesa: fix typos and add some format fallbacks in format_map

Always default to DEFAULT_*_FORMATS for mandatory GL formats.
   (st_choose_format must not fail for those)
Use DEFAULT_RGBA when alpha is required instead of RGB.
Use DEFAULT_RGB otherwise.

These are more or less the remaining differences between the old code and
the new one.

Reviewed-by: Brian Paul <brianp@vmware.com>