T-API: cv::morphologyEx

**Description:**
* eliminated needless `UMat::copyTo`
* merged last 2 kernels (morph + subtract -> morph) for `MORPH_GRADIENT`, `MORPH_TOPHAT`, `MORPH_BLACKHAT`
* ported changes from PR https://github.com/Itseez/opencv/pull/2867

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2899/report/

check_regression=*OCL*MorphologyEx*:*OCL*Erode*:*OCL*Dilate*
test_filter=*OCL*MorphologyEx*
test_modules=imgproc
build_examples=OFF