radv: use util_float_to_half_rtz
Since commit 8b8af6d3 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.