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
@KruchDmitriy  please, fix compiling warning
`warning C4267: 'initializing' : conversion from 'size_t' to 'int', possible loss of data [C:\builds_ocv\precommit_opencl\build\modules\imgproc\opencv_imgproc.vcxproj]`