T-API: some optimizaions of cv::pow

**Description:**
* used different built-in functions for particular cases depending on `power`

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2840/report/

check_regression=*OCL*Pow*
test_filter=*OCL*Pow*
test_modules=core
build_examples=OFF