OCL: Optimize OpenCL version of Laplacian filter for kernel size great than 3

Join two separateFilter in one kernel.

Report:
http://ocl.itseez.com/intel/export/perf/pr/3155/report/

check_regression=*OCL*Laplacian*
test_filter=*OCL*Laplacian*
test_modules=imgproc
build_examples=OFF