OCL: Optimization OpenCL version of Filter2D

Simplification of code. Change local size for intel devices.

Kernel was changed for `channels = 4` and `kernelSize >= 5`, only. Thus we look at **8UC4, 5** and **32FC4, 5** at perfomance report.

Report:
http://ocl.itseez.com/intel/export/perf/pr/3208/report/

check_regression=*OCL*Filter2D*
test_filter=*OCL*Filter2D*
test_modules=imgproc
build_examples=OFF