docs/panfrost: Document u-interleaved tiling

docs/panfrost: Document u-interleaved tiling

The optimized routine documented the tiling format together with the software
algorithm. The reference implementation wants the tiling format alone
documented. Let's break out the high level documentation into somewhere
centrally accessible, and refocus the comments in the optimized file on the
optimization.

This documentation is linked bidirectionally with both implementations, so it
should be easy to find.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15803>