panfrost: Fix non-debug builds

panfrost: Fix non-debug builds

For non-debug builds, where assertions are compiled out, GCC complains
about the end of the non-void function panfrost_translate_channel_width
being reached.

Fixes: 226c1efe9a8 ("panfrost: Add more info to some assertions")
Reported-by: Piotr Oniszczuk
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3665>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3665>