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/
@vbystricky Are 4 rows optimal and 8 rows are processed slowly or the same?