gallium: Format code in pb_buffer_fenced.c according to style guide.

gallium: Format code in pb_buffer_fenced.c according to style guide.

This is a tiny housekeeping patch which does the following:

  * Replaced tabs with three spaces.
  * Formatted oneline and multiline code comments. Some doxygen
    comments weren't marked as such and some code comments were marked
    as doxygen comments.
  * Spaces between if- and while-statements and their parenthesis.

According to the mesa coding style guidelines.

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