I managed to make it work somehow, but it's still unusable due to producing
artifacts on images processed using opencl on darktable and few other programs
I tried. So probably the title should be changed.

However, CL_OUT_OF_HOST_MEMORY error may be removed via envitonment var
setting, namely 

export GPU_FORCE_64BIT_PTR=1

also

export GPU_USE_SYNC_OBJECTS=1
export GPU_MAX_ALLOC_PERCENT=100
export GPU_SINGLE_ALLOC_PERCENT=100
export GPU_MAX_HEAP_SIZE=100

are recommended but I haven't noticed any difference.
What I've noticed, however, is that the driver reports the different amount of
available GPU memory each time darktable is started...