pan/va: Generate header containing enums

pan/va: Generate header containing enums

We already collect enums in the ISA description XML. Export them for use in the
compiler backend, particularly the packing code.

Usually we'd use Mako for templating. In this case, the script is so trivial a
template engine didn't seem worth it. (The obvious version with Mako was about
10 lines longer than just prints and f-strings used here.)

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Suggested-by: Icecream95 <ixn@disroot.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15223>