OCL: stitching warpers

- Attempt to optimize buildMaps kernels in case rowsPerWI > 1.
- Fixed call of PlaneWarper kernel

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/3359/report/

check_regression=*OCL*Warper*
test_filter=*OCL*Warper*
test_modules=stitching
build_examples=OFF
I tried to do calculation like this, but it doesn't improve performance. I prefer to leave as it is.