i965: Fix gen6,gen7 when used with a non-HiZ capable DDX

i965: Fix gen6,gen7 when used with a non-HiZ capable DDX

Nothing works if HiZ is enabled and the DDX is incapable of HiZ (that is,
the DDX version is < 2.16).

The problem is that the refactoring that eliminated
intel_renderbuffer::stencil_rb broke the recovery path in
intel_verify_dri2_has_hiz().  Specifically, it broke line
intel_context.c:1445, which allocates the region for
DRI_BUFFER_DEPTH_STENCIL. That allocation was creating a separate stencil
miptree, despite the buffer being a packed depthstencil buffer. Havoc
ensued.

This patch introduces a bool flag that prevents allocation of that stencil
miptree.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44103
Tested-by: Ian Romanick <idr@freedesktop.org>
Note: This is a candidate for the 8.0 branch.
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>