OCL: Optimize OpenCL LUT function

check_regression=*LUT*
test_modules=core

Local buffer was created and LUT table was copied into it. Also we rework 4 pixel in every thread.

http://ocl.itseez.com/intel/export/perf/pr/2755/report/
@krodyush Yes. I checked on HD4600 and 4 rows was better then 8 and better then 2.