gallium/util: Add back (and rename) util_float_to_half implementation

gallium/util: Add back (and rename) util_float_to_half implementation

This implementation was removed by 8b8af6d3 ("gallium/util: Switch
util_float_to_half to _mesa_float_to_half()'s impl.")
It was not actually broken, but _mesa_float_to_half() implements
round-to-nearest-even, whereas util_float_to_half() implemented
round-to-zero. So rename it appropriately.
GL actually never cares about rounding (except a broken piglit test),
however d3d10 very much does and requires RTZ for float to half
conversion. Moreover, apparently at least radeon gpus actually always
do RTZ when doing RT writes (and I'd suspect for shader image writes
as well). Hence it seems appropriate to hook up this rtz function to
the format instead. This will cause llvmpipe and softpipe to use rtz
rounding for clears with half float formats, and softpipe would use rtz
behavior for rt writes as well (llvmpipe has that hardcoded), not sure
if "real" hw drivers hit this function for much.
(For shader opcodes would still need to figure out what rounding to use
appropriately, but this is a question for another day.)
Note should probably unify with _mesa_float_to_float16_rtz. Unclear at
this point which one is better, so just restore previous function here.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4312>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4312>