i965/miptree: Use the isl helpers for creating aux surfaces

i965/miptree: Use the isl helpers for creating aux surfaces

In order for the calculations of things such as fast clear rectangles to
work, we need more details of the auxiliary surface to be correct.  In
particular, we need to be able to trust the width and height fields.
(These are not necessarily what you want coming out of the miptree.)  The
only values state setup really cares about are the row and array pitch and
those we can safely stomp from the miptree.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>