OCL: Optimize OpenCL version of conversScaleAbs function

1. Create `_dst` matrix before call `ocl::predictOptimalVectorWidth`
2. Change default vectorWidths in `ocl::predictOptimalVectorWidth` for this function

http://ocl.itseez.com/intel/export/perf/pr/3354/report/

check_regression=*OCL*ConvertScaleAbs*
test_filter=*OCL*ConvertScaleAbs*
test_modules=core
build_examples=OFF

ok