T-API: optimization of cv::cornerMinEigenVal

**Description:**
* eliminated quite useless temporary arrays (thanks to K.R. for advice)
* used built-in function `native_sqrt`

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2961/report/

check_regression=*OCL*Corner*
test_filter=*OCL*Corner*
test_modules=imgproc
build_examples=OFF