i965: Delete render ring prelude.

i965: Delete render ring prelude.

This was a hook I came up when trying to do the initial performance
counter work years ago.  Nothing's used it for a long time, and the
upcoming performance counter support doesn't want it either.

So, goodbye render ring prelude.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>