i965/cs: Initialize GPGPU Thread Count

i965/cs: Initialize GPGPU Thread Count

This field should always be set for gen8. In the bdw PRM, Volume 2d:
Command Reference: Structures under INTERFACE_DESCRIPTOR_DATA, DWORD
6, Bits 9:0, Number of Threads in GPGPU Thread Group:

"This field should not be set to 0 even if the barrier is disabled,
since an accurate value is needed for proper pre-emption."

In the HSW PRM, the it doesn't mention that it must always be set, but
it should not hurt.

Reported-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>