i965/cs: Don't use a thread channel ID for small local sizes

i965/cs: Don't use a thread channel ID for small local sizes

When the local group size is 8 or less, we will execute the program at
most 1 time. Therefore, the local channel ID will always be 0. By
using a constant 0 in this case we can prevent using push constant
data.

This is not expected to be common a occurance in real applications,
but it has been seen in tests.

We could extend this optimization to 16 and 32 for SIMD16 and SIMD32,
but it gets a bit more complicated, because this optimization is
currently being done early on, before we have decided the SIMD size.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>