i965: Fix S8 and X8 reversal in brw_depthbuffer_format refactor.

i965: Fix S8 and X8 reversal in brw_depthbuffer_format refactor.

In commit 09d9a8913e8c28fc4c1c60d7da85a2f093786894, I accidentally
botched the X8 and S8 cases.  (I wrote this patch before realizing that
X8 and S8 had been swapped in the big MESA_FORMAT rename, and apparently
didn't rebase it properly after fixing that...)

Fixes regressions in 13 Piglit tests on Ironlake.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75291
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>