OCL: fix for ocl_morphSmall

Performance report:
http://ocl.itseez.com/intel/export/perf/pr/3144/report/

check_regression=OCL*Erode*:OCL*Dilate*:OCL*Morph*:OCL*Box*
test_filter=OCL*Erode*:OCL*Dilate*:OCL*Morph*:OCL*Box*
build_examples=OFF
test_modules=imgproc