T-API: optimization of cv::remap

**Description:**
* increased number of rows per work-item
* used precomputed bufferof interpolation coefficients
* used `vload2` for `INTER_LINEAR`

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2888/report/

check_regression=*OCL*Remap*:*OCL*Warper*
test_modules=imgproc,stitching
test_filter=*OCL*Remap*
build_examples=OFF