i965/cs: Split out helper for building local id payload

i965/cs: Split out helper for building local id payload

The initial motivation for this patch was to avoid calling
brw_cs_prog_local_id_payload_dwords() in gen7_cs_state.c from the
compiler. This commit ends up refactoring things a bit more so as to
split out the logic to build the local id payload to brw_fs.cpp. This
moves the payload building closer to the compiler code that uses the
payload layout and makes it available to other users of the compiler.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Signed-off-by: Kristian Høgsberg Kristensen <krh@bitplanet.net>