i965/sf_state: Pull flat_enables out of prog_data

i965/sf_state: Pull flat_enables out of prog_data

Previously, we were walking over the shader source to figure out which
inputs should be marked flat.  Now, we can just pull it out of prog_data.
This is needed for properly setting up 3DSTATE_SF/SBE for Vulkan and it
also means that it will get properly cached.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>