radv: use util_float_to_half_rtz

radv: use util_float_to_half_rtz

Since commit 8b8af6d398a94cb07015c695fdfdb5c157aa72cf there is a
performance regression in dirt 4 on picasso APUs.

The game ends up feeding a large value into this which overflows on the
conversion to 16bit float. With the old implementation (which now lives
in util_float_to_half_rtz) it would be clamped to inf-1, while the new
one returns inf. This causes a performance hit somehow at some point
down the line.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Fixes: 8b8af6d398a "gallium/util: Switch util_float_to_half to _mesa_float_to_half()'s impl."
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5062>