i965: Always handle GL_DEPTH_TEXTURE_MODE through the shader.

i965: Always handle GL_DEPTH_TEXTURE_MODE through the shader.

We were already doing it through the shader (layered underneath
GL_EXT_texture_swizzle) in the shadow compare case.  This avoids
having per-format logic for switching out the surface format dependent
on the depth mode.

v2: Also do the swizzling for DEPTH_STENCIL.  oops.

Reviewed-by: Ian Romanick <idr@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>