mesa: Expose ATI_draw_buffers.

mesa: Expose ATI_draw_buffers.

This is the same as ARB_draw_buffers (which derived from it), except
for s/ARB/ATI/.  The glapi bits were already in place, and what was
missing was just the ARB_fp part.  The new Humble Bundle game "trine"
tries to use this extension without checking that it's exposed, which
this works around.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36182
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>