util/format_rgb9e5: Get rid of the rgb9e5 union

The rgb9e5 format is a packed format defined in terms of slicing up a
single 32-bit value.  The bitfields are far more confusing than simple
shifts and require that we check the endianness.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>