OCL: Optimize OpenCL version function BackgroundSubstractionMOG2

Remove some useless prameters
Change using ShadowDetect parameters to build options
Use mad/mad24 in all places

Perfomance report:
http://ocl.itseez.com/intel/export/perf/pr/3223/report/

check_regression=*OCL*MOG2*
test_filter=*OCL*Mog2*
test_modules=video
build_examples=OFF
Yes, i changed ``_modesUsed `` from ``int * `` to ``uchar *`` and performance improved.
Using ``int`` instead of ``char`` for ``mode`` and ``nmodes`` don't change perfomance (on my workstation)