util/rgb9e5: Get rid of the float754 union

util/rgb9e5: Get rid of the float754 union

There are a number of reasons for this refactor.  First, format_rgb9e5.h is
not something that a user would expect to define such a generic union.
Second, defining it requires checking for endianness which is ugly.  Third,
90% of what we were doing with the union was float <-> uint32_t bitcasts
and the remaining 10% can be done with a sinmple left-shift by 23.

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