i965/fs: Move brw_wm_compile::dispatch_width into fs_visitor.

i965/fs: Move brw_wm_compile::dispatch_width into fs_visitor.

Also, rather than having brw_wm_fs_emit poke at it directly, make it a
parameter to the fs_visitor constructor.

All other changes generated by search and replace (with occasional
whitespace fixup).

v2: Make dispatch_width const (as suggested by Paul); fix doxygen
    mistake (pointed out by Eric); update for rebase.

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