i965/fs: Remove struct brw_shader * parameter to fs_visitor constructor.

i965/fs: Remove struct brw_shader * parameter to fs_visitor constructor.

We can easily recover it from prog, and this makes it clear that we
aren't passing additional information in.

v2: Use an if-statement rather than the ?: operator (suggested by Eric).

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>