util: Move gallium's PIPE_FORMAT utils to /util/format/

util: Move gallium's PIPE_FORMAT utils to /util/format/

To make PIPE_FORMATs usable from non-gallium parts of Mesa, I want to
move their helpers out of gallium.  Since u_format used
util_copy_rect(), I moved that in there, too.

I've put it in a separate directory in util/ because it's a big chunk
of related code, and it's not clear to me whether we might want it as
a separate library from libmesa_util at some point.

Closes: #1905
Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>