i965: Move payload reg setup to compile, not lookup time.

i965: Move payload reg setup to compile, not lookup time.

Payload reg setup on gen6 depends more on the dispatch width as well
as the uses_depth, computes_depth, and other flags.  That's something
we want to decide at compile time, not at cache lookup.  As a bonus,
the fragment shader program cache lookup should be cheaper now that
there's less to compute for the hash key.