OCL: minor optimization for Canny

Attempt to optimize `stage1_with_sobel` kernel using float calculations instead integer.

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/3394/report

check_regression=*OCL*Canny*
test_filter=*OCL*Canny*
test_modules=imgproc
build_examples=OFF
a cycle with unknown number of iterations would not be unrolled.