mesa: Skip 3-byte array formats in _mesa_array_format_flip_channels

mesa: Skip 3-byte array formats in _mesa_array_format_flip_channels

Byte swapping makes no sense for 3-byte formats: Swapping the order of 2
or 4 bytes at a time would inevitably result in bytes getting mixed up
between neighbouring pixels.

Fixes crash with a debugging build on a big endian machine due hitting
the unreachable() at the end of the function.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2665
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4735>