ocl: Move static oclMat variables into FarnebackOpticalFlow class

Move some static functions into the FarnebackOpticalFlow class as well,
so they can access these new class variables.

oclMat objects cannot be declared statically, because their destructor
depends on the statically defined __module variable from cl_context.cpp.
Since statically defined variables in separate compilation units have
an undefined destruction order there is always the possibility the
__module will be destructed before an oclMat object, which results
in a segfault.
Merge pull request #3427 from tstellarAMD:2.4-farneback-crash-fix