T-API: optimized cv::calcHist, cv::equalizeHist

**Description:**
* changed data chunk for iteration of cycle
* changed `uchar4` -> `int` since it's faster to load
* eliminated `convertTo` in the last stage of the algo
* optimized histogram merging
* merged 2 kernels into one in `cv::equalizeHist`: `merge_histogram` + `calcLUT` -> `calcLUT`

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2840/report/

check_regression=*OCL*Hist*
test_filter=*OCL*Hist*
test_modules=imgproc
build_examples=OFF