OCL: DFT fix

Attempt to stabilize performance tests results for DFT: changed twiddle buffer creation to use OCL buffer pool.

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/3121/report/

check_regression=*OCL*Dft*
test_filter=*OCL*Dft*
test_modules=core
build_examples=OFF