OCL: small fixes for Laplacian, Canny, etc

Also fixed extrapolation in sepFilter2D and eigenValsVecs.

check_regression=OCL*Laplacian*:OCL*Canny*:OCL*Corner*:OCL*Sobel*:OCL*GaussianBlur*:OCL*Scharr*
test_filter=OCL*Laplacian*:OCL*Canny*:OCL*Corner*:OCL*Sobel*:OCL*GaussianBlur*:OCL*Scharr*
build_examples=OFF
test_modules=imgproc
@akarsakov, Do we really need this `clamp`? I think check in host code: 
```(_src.cols() >= kernelX.cols && _src.rows() >= kernelY.cols)```
fix this case.