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
Did you try to use `int` instead of `char` type (here and for nmodes)?