i965: Disable the swrast context setup on GL 3.1 core.

i965: Disable the swrast context setup on GL 3.1 core.

I've reviewed the code, and the swrast callsites remaining are all in
drawpixels/copypixels/bitmap/accum, or _swrast_BlitFramebuffer that shouldn't
be hit.  A piglit run with the context setup disabled on legacy GL and GLES2
showed regressions only in the copypixels and drawpixels tests.

If the context type is forced, this reduces the shader_runner maximum heap
size for glsl-algebraic-add-add-1.shader_test from 15,137,496b to 4,165,376b.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>