i965: Implement ARB_stencil_texturing on Gen8+.

On earlier hardware, we had to implement math in the shader to translate
Y-tiled or untiled coordinates to W-tiled coordinates (which is what
BLORP does today in order to texture from stencil buffers).

On Broadwell, we can simply state that it's W-tiled in SURFACE_STATE,
and adjust the pitch.  This is much easier.

In the surface state code, I chose to handle the "should we sample depth
or stencil?" question separately from the setup for sampling from
stencil.  This should make it work with the BindRenderbufferTexImage
hook as well, and hopefully be reusable for GL_ARB_texture_stencil8
someday.

v2: Update docs/GL3.txt (caught by Matt).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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