ocl::BruteForceMatcher fix wrong use of oclMat::ptr().

The oclMat::ptr() method was mistakenly used in ocl::BruteForceMatcher to pass a pointer to a oclMat object. The ptr() method returns a uchar pointer to the cl_mem data structure and this method will be removed.
Merge pull request #1940 from pentschev:ocl_bfm_oclMat_ptr_fix_2.4