OCL: Denoising fix

Minimal image size is set to 5*5 for OCL path of `fastNlMeansDenoising` and `fastNlMeansDenoisingColored` since it breaks tests with `test_loop_times=30` and in general doesn't make sense for computation on GPU.

test_modules=photo
build_examples=OFF
check_regression=*OCL*Denoising*
test_filter=*OCL*Denoising*