i965/cs: Use override structure rather than separate env var

i965/cs: Use override structure rather than separate env var

In 25268b93, we added a new environment variable
(INTEL_COMPUTE_SHADER) to allow some constant values to be upgraded
for the ARB_compute_shader extension.

Now, we can look to see if the extension was enabled via the
MESA_EXTENSION_OVERRIDE environment variable.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>