The response in this thread is still relevant:
Intel Iris Graphics and Intel HD Graphics do not support the cl_khr_fp64 extension.
Generally for fp64 usage, our majority of users are using Intel Xeon processors or Intel Xeon Phi coprocessors (vs low power devices like the intel processor graphics)
Thanks for your feedback on your interest in double precision floating point for Intel processor graphics. However, this is a hardware limitation and not an easy thing to add.