i965: Move loop over texture units into brw_populate_sampler_prog_key.

i965: Move loop over texture units into brw_populate_sampler_prog_key.

The whole reason I avoided this was because it might operate on a
brw_vertex_program or a brw_fragment_program.  However, that isn't a
problem: all we need is the gl_program base type.

This avoids awkwardly passing the loop counter 'i' as a parameter,
simplifies both callers, and also plumbs prog in place for future use.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>