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
For multiple kernels you can use this approach:
```
... common code (WSZ2?) ...

#ifdef DEFINE_KERNEL_stereoBM
__kernel stereoBM(...)
...
#endif

#ifdef DEFINE_KERNEL_prefilter_norm
... code without any BLOCK_SIZE_X/Y ...
#endif
```