i965: only upload constant buffer data when we actually need the const buffer

i965: only upload constant buffer data when we actually need the const buffer

Make the use_const_buffer field per-program and only call the code which
updates the constant buffer's data if the flag is set.

This should undo the perf regression from 20f3497e4b6756e330f7b3f54e8acaa1d6c92052

(cherry picked from master, commit dc9705d12d162ba6d087eb762e315de9f97bc456)