i965: Make brw_update_sampler_state() use brw_emit_sampler_state().

i965: Make brw_update_sampler_state() use brw_emit_sampler_state().

Instead of stuffing bits directly into the brw_sampler_state structure,
we now store them in local variables, then use brw_emit_sampler_state()
to assemble the packet.  This separates the decision about what values
to use from the actual packet emission, which makes the code more
reusable across generations.

v2: Put const on a bunch of local variables and move declarations,
    as suggested by Topi Pohjolainen.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>