Revert "gallium/util: Switch util_float_to_half to _mesa_float_to_half()'s impl."

This reverts commit 8b8af6d3.

We apparently have a requirement that the version in gallium does round-to-zero due to DX10 requirements for framebuffer writes. This function being the pack function for u_format makes that happen for llvmpipe. Leave a comment for the next person that stumbles across this.