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
@vbystricky This is for case `x > 2*maxV`. This can happen when the input matrix have small size and size of work group is rounded up.
Maybe it's not the best solution,  but it always guarantees correct result. What do you suggest?