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

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

The util_float_to_half() implementation was much smaller, but when trying
to switch _mesa_float_to_half to it, many testcases
(dEQP-VK.spirv_assembly.instruction.graphics.opquantize.*,
piglit.spec.arb_shading_language_packing.*packhalf2x16) start failing on
Intel.  Replace the broken impl so that people don't have to debug it
later.

Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3699>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3699>