i965: Enable ARB_compute_shader extension on supported hardware

i965: Enable ARB_compute_shader extension on supported hardware

Enable ARB_compute_shader on gen7+, on hardware that supports the
OpenGL 4.3 requirements of a local group size of 1024.

With SIMD16 support, this is limited to Ivy Bridge and Haswell.

Broadwell will work with a local group size up to 896 on SIMD16
meaning programs that use this size or lower should run when setting
MESA_EXTENSION_OVERRIDE=GL_ARB_compute_shader.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>