Darktable's OpenCL isn't activated by default on Fedora when using the RPMFusion Nvidia drivers, apparently because Darktable can't find the library:

$ darktable -d opencl
(...)
[opencl_init] trying to load opencl library: '<system default>'
[opencl_init] could not find opencl runtime library 'libOpenCL'
[opencl_init] no working opencl library found. Continue with opencl disabled
[opencl_init] FINALLY: opencl is NOT AVAILABLE on this system.
[opencl_init] initial status of opencl enabled flag is OFF.

A quick, if hacky, solution is to just make a symlink to Darktable's directory so it can find the library:

# ln -s /usr/lib64/nvidia-304xx/libOpenCL.so.1 /usr/lib64/darktable/libOpenCL.so

(Obviously, adjust the paths based on your system.)