i965: fixup W-tile offset computation to take swizzling into account

i965: fixup W-tile offset computation to take swizzling into account

There's even a comment in the code containing the right swizzling
computations!

Previously this has not been noticed because we need to manually
enabled swizzling on snb/ivb (kernel 3.4 will do that) and we
don't use the separate stencil on ilk (where the bios enables
swizzling). This fixes

piglit ./bin/fbo-stencil  readpixels GL_DEPTH32F_STENCIL8 -auto

on recent drm-intel-next kernels.

Also remove the comment about ivb, it's stale now.

Swizzling detection is done by allocating a temporary x-tiled
buffer object. Unfortunately kernels before v3.2 lie on snb/ivb
because they claim that swizzling is enable, but it isn't. The
kernel commit that fixes this for backport to pre-v3.2 is

commit acc83eb5a1e0ae7dbbf89ca2a1a943ade224bb84
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Sep 12 20:49:16 2011 +0200

    drm/i915: fix swizzling on gen6+

But if the kernel doesn't lie, this now works on swizzling and
not swizzling machines.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>