intel: Fix stencil buffer to be W tiled

intel: Fix stencil buffer to be W tiled

Until now, the stencil buffer was allocated as a Y tiled buffer, because
in several locations the PRM states that it is. However, it is actually
W tiled. From the PRM, 2011 Sandy Bridge, Volume 1, Part 2, Section
4.5.2.1 W-Major Format:
    W-Major Tile Format is used for separate stencil.

The GTT is incapable of W fencing, so we allocate the stencil buffer with
I915_TILING_NONE and decode the tile's layout in software.

This fix touches the following portions of code:
    - In intel_allocate_renderbuffer_storage(), allocate the stencil
      buffer with I915_TILING_NONE.
    - In intel_verify_dri2_has_hiz(), verify that the stencil buffer is
      not tiled.
    - In the stencil buffer's span functions, the tile's layout must be
      decoded in software.

This commit mutually depends on the xf86-video-intel commit
    dri: Do not tile stencil buffer
    Author: Chad Versace <chad@chad-versace.us>
    Date:   Mon Jul 18 00:38:00 2011 -0700

On Gen6 with separate stencil enabled, fixes the following Piglit tests:
    bugs/fdo23670-drawpix_stencil
    general/stencil-drawpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX16-copypixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX16-drawpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX16-readpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX1-copypixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX1-drawpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX1-readpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX4-copypixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX4-drawpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX4-readpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX8-copypixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX8-drawpixels
    spec/EXT_framebuffer_object/fbo-stencil-GL_STENCIL_INDEX8-readpixels
    spec/EXT_packed_depth_stencil/fbo-stencil-GL_DEPTH24_STENCIL8-copypixels
    spec/EXT_packed_depth_stencil/fbo-stencil-GL_DEPTH24_STENCIL8-readpixels
    spec/EXT_packed_depth_stencil/readpixels-24_8

Note: This is a candidate for the 7.11 branch.

Signed-off-by: Chad Versace <chad@chad-versace.us>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>