OCL: Improve the performance of fast_nlmeans_denoising_opencl.

1. Remove unnecessary barriers.
2. Adjust CTA_SIZE based on the following cases:
   a) OCL_Photo_DenoisingGrayscale.DenoisingGrayscale
   b) OCL_Photo_DenoisingColored.DenoisingColored

check_regression=*OCL*
test_modules=photo
test_filter=*OCL*
build_examples=OFF