mesa/formats: Don't flip channels of null array formats
Before, if we encountered an array format of 0 on a BE system, we would flip all the channels even though it's an invalid format. This would result in a mostly invalid format with a swizzle of yyyy or wwww. Instead, we should just return 0 if the array format stashed in the format info is invalid. Cc: "10.6 10.5" <mesa-stable@lists.freedesktop.org> (cherry picked from commit e3eb91af)