OCL: Optimize OpenCL version of StereoBM function

1. Fix problems on NVidia devices.
2. Move some arguments to build options
3. Refactoring some code.

http://ocl.itseez.com/intel/export/perf/pr/3284/report/

check_regression=*OCL*StereoBM*
test_filter=*OCL*StereoBM*
test_modules=calib3d
build_examples=OFF
I commented all calculation in `if (x < cols - WSZ2 - MIN_DISP && y < rows - WSZ2) ... else ...` and checked time. Perfomance increase around 2-5%.