panfrost: Use C11 static_assert for enums

Rather than asserting everything in an unused function, just do it in global
context with C11 static_asserts. This is a bit neater now that we depend on C11
projectwide.

Obvious follow-on from !16670.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <!16856>