i965/miptree: Add new clear color BO for winsys aux buffers

i965/miptree: Add new clear color BO for winsys aux buffers

Add an extra BO to store clear color when we receive the aux buffer from
the window system. Since we have no control over the aux buffer size in
this case, we need the new BO to store only the clear color.

v5:
 - Better subject (Jordan).
 - Drop alignment from brw_bo_alloc().

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