OCL: fixed Erode and Dilate in case of kernel = Mat()

* fixed Erode and Dilate in case of kernel = Mat(),  found in performance test OCL_stitch_a123
* added accuracy tests for kernel = Mat()

test_modules=imgproc
test_filter=OCL*Erode*:OCL*Dilate*
check_regression=OCL*Erode*:OCL*Dilate*
build_examples=OFF