OCL: Optimize ocl version of warp_affine

Small changes for interpolation type = `INTER_NEAREST`

check_regression=*OCL*WarpAffine*
test_filter=*OCL*WarpAffine*
test_modules=imgproc
build_examples=OFF

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2993/report/