This is to add fill operation using GPGPU pipeline which is similar to
current media fill. This can be used to simply verify GPGPU pipeline
and help to enable it on newer HW, currently it works on Gen7 only and
will add support on later platform.

Now this sets very simply thread group dispatch for one thread per
thread group on SIMD16 dispatch. So the fill shader just uses thread
group ID for buffer offset.

v2: No new fill func typedef but adapt to igt_fillfunc_t.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>