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/
Not always. dstT can be int, float etc.
I changed code and checked it for uchar. Performance not increases on my hardware.