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