OCL: Optimize OpenCL version of meanStdDev

**Description:**
* Change groups count for Intel.

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2935/report/

check_regression=*OCL*MeanStdDev*
test_filter=*OCL*MeanStdDev*
test_modules=core
build_examples=OFF