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: 226c1efe9a ("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>

src/gallium/drivers/panfrost/pan_format.c

@@ -101,6 +101,7 @@ panfrost_translate_channel_width(unsigned size)
         default: {
                 fprintf(stderr, "Invalid width: %d\n", size);
                 assert(0);
+                return 0;
         }
         }
 }