ir3: Use round-to-nearest-even for fquantize2f16

We're supposed to map a floating-point value too large to be represented as fp16 to infinity, however round-to-zero naturally rounds it down to the largest representable fp16 number instead. The blob emits a bunch of fixup code to work around this, but instead we can just do what all the other drivers seem to do and use round-to-nearest-even instead.