OCL: performance test for ocl_gemm

check_regression=OCL*Gemm*
test_filter=OCL*Gemm*
test_modules=core
build_examples=OFF
Merge with extra: https://github.com/Itseez/opencv_extra/pull/214
these are quite strange numbers for matrix multiplication; what was the problem with 1000x1000 and 1500x1500?