OCL: optimization of pyrDown for 8UC1

**Description**
- used fma for sum calculating
- loading data using uchar4 for 8UC1

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/2882/report/

check_regression=*OCL*yrDown*
test_filter=*OCL*yrDown*
build_examples=OFF
test_modules=imgproc