util/format: nr_channels is always <= 4

While the nr_channels is defined with 3 bits, which allows up to 7
channels, actually the number of channels is less or equal to 4.

This adds an assertion that helps static analyzers to avoid several
false positives related with this.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32589>

src/util/format/u_format.h

@@ -1361,6 +1361,8 @@ static inline unsigned
 util_format_get_nr_components(enum pipe_format format)
 {
    const struct util_format_description *desc = util_format_description(format);
+   assert(desc->nr_channels <= 4);
+
    return desc->nr_channels;
 }