i965/brw: Fix emit_depthbuffer() when packed depth/stencil texture is attached

i965/brw: Fix emit_depthbuffer() when packed depth/stencil texture is attached

If either depth or stencil buffer has packed depth/stencil format, then do
not use separate stencil.

Before this commit, emit_depthbuffer() incorrectly assumed that the
texture's stencil renderbuffer wrapper was a *separate* stencil buffer,
because the depth and stencil renderbuffer wrappers are distinct for
depth/stencil textures (that is, depth_irb != stencil_irb).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38134
Signed-off-by: Chad Versace <chad@chad-versace.us>