dri/nouveau: replace assert with unreachable

dri/nouveau: replace assert with unreachable

I don't know why removing imports.h suddenly makes clang realize that
this function can not return in a non-debug build, but it does.
Unreachable is better because it doesn't have this problem.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3024>