OpenCL requires AMDGPU-PRO and that AUR package has been orphaned.
You don't need the full set of amdgpu-pro drivers - I use the in kernel amdgpu driver with the following AUR package, which isn't orphaned:
aur/opencl-amd 17.40.492261-1 (34, 5.12)
OpenCL userspace driver as provided in the amdgpu-pro driver stack. This package is intended to work along with the free
amdgpu stack.