src/util: Switch _mesa_half_to_float() to u_half.h's version.

src/util: Switch _mesa_half_to_float() to u_half.h's version.

The two implementations differ across the entire input range only in
that u_half.h preserves mantissa bits for NaNs.  The u_half.h version
shaves 15% off of the text size of half_float.o.

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>