docs/tgsi: fix up indent

docs/tgsi: fix up indent

There's a mixture of indent styles here, with either two or three
spaces. We have standardized on three spaces for .rst-files in the
editorconfig, so let's apply that.

While we're at it, make sure math-blocks are indented into their
opcode-block. While the result might look the same most of the time,
this matters when we have textual explaination following math-blocks,
like we have in a few caess. If we don't indent the math there, we
end up with having to unindent the text following the math-block for it
not to count as a part of the math block, which looks very confusing
when reading the source code.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21893>