OCL: separate kernel for small SE in MorphologyEx, Erode,Dilate

**WIP**
Optimization: optimization effort for morph kernel, the same as in filter2D

Current issue - not finished for TOPHAT, BLACKHAT and GRADIENT modes, working correctly for ERODE, DILATE, OPEN and CLOSE. First 3 modes need to be fixed in optimized version.

Performance report:
http://ocl.itseez.com/intel/export/perf/pr/2981/report/

check_regression=*OCL*Erode*:*OCL*Dilate*:*OCL*Morph*
build_examples=OFF
test_modules=imgproc
test_filter=*OCL*Erode*:*OCL*Dilate*:*OCL*Morph*
`matptr` must have its own `mat_index` since `mat_offset` is not equal to `dst_offset` (the same for steps)