mesa/formats: Don't flip channels of null array formats

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>