T-API: optimized cv::setIdentity

**Description:**
* used vector data types for `8u`, `8s`

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2851/report/

check_regression=*OCL*SetIden*
test_filter=*OCL*SetIden*
test_modules=core
build_examples=OFF