i965: Add aux_buf variable to simplify code.

i965: Add aux_buf variable to simplify code.

In a follow up patch, we make use of clear_color_bo, which is in
mt->mcs_buf or mt->hiz_buf. To avoid duplicating more code that does the
same thing on both aux buffers, just use aux_buf already.

v5: Add aux_buf to brw_wm_surface_state too.
v6: Drop aux_surf and use aux_buf->surf instead (Jason).

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>