i965: Use the separate stencil buffer's offsets for stencil setup.

i965: Use the separate stencil buffer's offsets for stencil setup.

For a packed depth/stencil buffer on separate stencil hardware, the
separate depth miptree is set up with alignment of 4,4 and the separate
stencil miptree is setup with alignment of 8,8.  We can't just use the
irb->draw_{x,y} offsets for stencil, since that is the offset in the
depth miptree.

Fixes 12 piglit depthstencil testcases on ivb.

Acked-by: Chad Versace <chad.versace@linux.intel.com>