i965: Add an INTEL_DEBUG=cs option.

i965: Add an INTEL_DEBUG=cs option.

At the moment it's not wired up to anything.  Later patches will hook
it up to the compute shader back-end.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>