OCL: Optimize OpenCL version of sepFilter2D

Don't merge. For test only.

check_regression=*OCL*Sobel*:*OCL*Scharr*:*OCL*GaussianBlur*
test_filter=*OCL*Sobel*:*OCL*Scharr*:*OCL*GaussianBlur*
test_modules=imgproc
build_examples=OFF