OCL: Optimization for canny

Performance optimization for canny.

http://ocl.itseez.com/intel/export/perf/pr/3117/report/

check_regression=OCL*Canny*
test_filter=OCL*Canny*
build_examples=OFF
test_modules=imgproc
does it make sense to use special sobel kernel for 5x5 and 7x7 aperture as it did int extractCovdata function (corner.cpp)?